Tft substrate with variable dielectric thickness

ABSTRACT

A transistor includes a substrate and an electrically conductive gate over the substrate. The gate has a gate length. A source electrode and a drain electrode are over the substrate, and are separated by a gap defining a channel region. The channel region has a channel length that is less than the gate length. A semiconductor layer is in contact with the source electrode and drain electrode. A dielectric stack is in contact with the gate, and has first, second, and third regions. The first region is in contact with the semiconductor layer in the channel region, and has a first thickness. The second region is adjacent to the first region that has the first thickness. The third region is adjacent to the second region, and has a thickness that is greater than the first thickness.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, U.S. patent application Ser. No.______ (Docket K001872), entitled “ENHANCEMENT-DEPLETION MODE INVERTERWITH TWO TRANSISTOR ARCHITECTURES”, Ser. No. ______ (Docket K001895),entitled “METHOD FOR FORMING A VARIABLE THICKNESS DIELECTRIC STACK”,Ser. No. ______ (Docket K001897), entitled “ENHANCEMENT MODE INVERTERWITH VARIABLE THICKNESS DIELECTRIC STACK”, all filed concurrentlyherewith.

FIELD OF THE INVENTION

This invention relates generally to patterned thin film fabrication andelectronic and optoelectronic devices including patterned thin films. Inparticular, this invention relates to selective area deposition ofmaterials including, for example, metal-oxides and devices including,for example, thin film transistors, inverters and circuits producedusing, for example, this fabrication technique.

BACKGROUND OF THE INVENTION

Modern-day electronics require multiple patterned layers of electricallyor optically active materials, sometimes over a relatively largesubstrate. Electronics such as radio frequency identification (RFID)tags, photovoltaics, optical and chemical sensors all require some levelof patterning in their electronic circuitry. Flat panel displays, suchas liquid crystal displays or electroluminescent displays (for example,OLED), rely upon accurately patterned sequential layers to form thinfilm components of the backplane. These components include capacitors,transistors, and power buses. The industry is continually looking fornew methods of materials deposition and layer patterning for bothperformance gains and cost reductions. Thin film transistors (TFTs) maybe viewed as representative of the electronic and manufacturing issuesfor many thin film components. TFTs are widely used as switchingelements in electronics, for example, in active-matrix liquid-crystaldisplays, smart cards, and a variety of other electronic devices andcomponents thereof.

There is a growing interest in depositing thin film semiconductors onplastic or flexible substrates, particularly because these supports aremore mechanically robust, lighter weight, and allow more economicalmanufacturing, for example, by allowing roll-to-roll processing.Plastics, however, typically limit device processing to below 200° C.There are other many issues associated with plastic supports when usingtraditional photolithography during conventional manufacturing, makingit difficult to perform alignments of transistor components acrosstypical substrate widths up to one meter or more. Traditionalphotolithographic processes and equipment may be seriously impacted bythe substrate's maximum process temperature, solvent resistance,dimensional stability, water, and solvent swelling, all key parametersin which plastic supports are typically inferior to glass.

The discovery of practical inorganic semiconductors as a replacement forcurrent silicon-based technologies has also been the subject ofconsiderable research efforts. For example, metal oxide semiconductorsare known that constitute zinc oxide, indium oxide, gallium indium zincoxide, tin oxide, or cadmium oxide deposited with or without additionaldoping elements including metals such as aluminum. Such semiconductormaterials, which are transparent, can have additional advantages forcertain applications. Additionally, metal oxide dielectrics such asalumina (Al₂O₃) and TiO₂ are useful in practical electronicsapplications as well as optical applications such as interferencefilters. Dielectric materials that are easily processable andpatternable are also important to the success of low cost and flexibleelectronic devices. In addition, metal oxide materials can serve asbarrier or encapsulation elements in various electronic devices. Thesematerials also require patterning so that a connection can be made tothe encapsulated devices.

Atomic layer deposition (ALD) can be used as a fabrication step forforming a number of types of thin-film electronic devices, includingsemiconductor devices and supporting electronic components such asresistors and capacitors, insulators, bus lines, and other conductivestructures. ALD is particularly suited for forming thin layers of metaloxides in the components of electronic devices. General classes offunctional materials that can be deposited with ALD include conductors,dielectrics or insulators, and semiconductors. A number of devicestructures can be made with the functional layers described above.

There is growing interest in combining ALD with a technology known asselective area deposition (SAD). As the name implies, selective areadeposition involves treating portion(s) of a substrate such that amaterial is deposited only in those areas that are desired, or selected.The majority of SAD work to date has focused on the problem ofpatterning a single material during deposition. There persists a problemof combining multiple SAD steps to form working devices, Processes forbuilding complete devices need to be able to control the properties thecritical interfaces, particularly in field effect devices like TFTs.

The majority of electronic devices require a high quality dielectriclayer. Although there are many approaches to forming a high qualitydielectric layer, they typically fall into one of two categories: asingle thick layer of a single material or multiple layers of differingmaterial types. In the case of devices which use a single layerdielectric, large thicknesses are required for defect mitigation toensure high device yield. This required layer thickness typicallyrequires long processing times and limits the functionality of fieldeffect devices. Devices formed with a multilayer stack of materials usethin layers of materials deposited using the same equipment requiringcomplex equipment design and multiple precursors. Accordingly, therestill remains a need for a high quality dielectric that can be formedfrom a single material for ease of processing and single precursors, andthat doesn't require a thick layer for performance and device yield.Additionally, a method is needed to simply pattern this layer for easydevice integration.

A particularly useful electronic device in building functional circuitryis an inverter, which functions to invert the polarity of an inputsignal. In CMOS circuitry, inverters are typically easy to design butdisadvantageously expensive to produce and utilize complicatedproduction processes. It is possible to build all NMOS or PMOSinverters, however particularly for enhancement-depletion mode circuitsthere are challenges to independently controlling the behavior of eachtransistor in the inverter circuit. Typically, the depletion modetransistor will have a thicker semiconductor layer than the enhancementmode transistor, increasing process complexity and increasing cost.Other alternatives include using dual gate architectures or multilayersemiconductor stacks, which have similar issues of process complexityand cost. As the industry endeavors to build circuitry using printingmethods, individual transistor size has a direct impact on the overallcircuit footprint on something like a label, as the individual componenttransistors are sized using their channel dimensions. There remains aneed to build high quality inverters using simple processes, byemploying novel architectures to control transistor, and therefore,circuit performance.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a transistor includes asubstrate and an electrically conductive gate over the substrate. Thegate has a gate length. A source electrode and a drain electrode areover the substrate, and are separated by a gap defining a channelregion. The channel region has a channel length that is less than thegate length. A semiconductor layer is in contact with the sourceelectrode and drain electrode. A dielectric stack is in contact with thegate, and has first, second, and third regions. The first region is incontact with the semiconductor layer in the channel region, and has afirst thickness. The second region is adjacent to the first region thathas the first thickness. The third region is adjacent to the secondregion, and has a thickness that is greater than the first thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the example embodiments of the inventionpresented below, reference is made to the accompanying drawings, inwhich:

FIGS. 1a and 1b are a cross-sectional view and a plan view,respectively, of an example embodiment of a bottom gate TFT including anexample embodiment of a variable thickness dielectric stack of thepresent invention;

FIG. 2 is a cross-sectional view of an embodiment of a bottom gate TFThaving a buffer layer and the variable thickness dielectric stack of thepresent invention;

FIG. 3 is a cross-sectional view of an embodiment of a bottom gate TFTwith a buffer layer included in an example embodiment of a variablethickness dielectric stack of the present invention;

FIG. 4a is a plan view of an example embodiment of a bottom gate TFTincluding an example embodiment of a variable thickness dielectric stackof the present invention;

FIGS. 4b and 4c are expanded plan view and cross sectional view of theregion near the channel region shown in FIG. 4 a;

FIG. 5 is a flow chart describing the steps of an example embodiment ofa process for forming the variable thickness dielectric;

FIGS. 6a through 6i are cross-sectional side views of an exampleembodiment of the process, described in FIG. 5, of forming the patternedvariable thickness dielectric;

FIG. 7 is a flow chart describing the steps of an example embodiment ofa process for forming a bottom gate TFT with a variable thicknessdielectric;

FIGS. 8a and 8b through 20a and 20b are cross-sectional side views andplan views, respectively, of an example embodiment of the process,described in FIG. 7, of forming a bottom gate TFT with a patternedvariable thickness dielectric;

FIG. 21 is an equivalent circuit for an all-enhancement-mode inverter;

FIGS. 22a and 22b are a plan view and a cross-sectional view,respectively, of an example embodiment of an inverter using a drive TFThaving the variable thickness dielectric stack of the present invention;

FIGS. 23a and 23b are cross-sectional views of a top-gate TFT;

FIG. 23c is a plan view of the top-gate TFTs shown in FIGS. 23a and 23b;

FIG. 24 is an equivalent circuit for an enhancement-depletion modeinverter;

FIGS. 25a and 25b are a plan view and a cross-sectional view,respectively, of an example embodiment of the present invention havingan inverter using a bottom-gate drive TFT and a top-gate load TFT;

FIGS. 26a and 26b are a plan view and a cross-sectional view,respectively, of one embodiment of the present invention of an inverterusing a bottom-gate drive TFT having the variable thickness dielectricstack and a top-gate load TFT;

FIG. 27 is a flow chart describing the steps of an example embodiment ofa process for forming an enhancement-depletion mode inverter with adrive TFT having a variable thickness dielectric;

FIG. 28 is a cross-sectional side view of a deposition device used forthin film deposition in the Examples; and

FIG. 29 is a graph illustrating the relationship between gate voltageand drain current for a 500 A gate oxide transistor of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The meaning of “a,” “an,” and “the” includes pluralreference, the meaning of “in” includes “in” and “on.” Additionally,directional terms such as “on”, “over”, “top”, “bottom”, “left”, “right”are used with reference to the orientation of the Figure(s) beingdescribed. Because components of embodiments of the present inventioncan be positioned in a number of different orientations, the directionalterminology is used for purposes of illustration only and is in no waylimiting.

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, an apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown, labeled, or described can take variousforms well known to those skilled in the art. In the followingdescription and drawings, identical reference numerals have been used,where possible, to designate identical elements. It is to be understoodthat elements and components can be referred to in singular or pluralform, as appropriate, without limiting the scope of the invention.

The example embodiments of the present invention are illustratedschematically and are not to scale for the sake of clarity. One ofordinary skill in the art will be able to readily determine the specificsize and interconnections of the elements of the example embodiments ofthe present invention. Therefore, the figures provided are not drawn toscale but are intended to show overall function and the structuralarrangement of some embodiments of the present invention.

Even though specific embodiments of the invention have been describedherein, it should be noted that the present invention is not limited tothese embodiments. In particular, any features described with respect toone embodiment may also be used in other embodiments, where compatible.The features of the different embodiments can be exchanged, wherecompatible.

For the description that follows, the term “gas” or “gaseous material”is used in a broad sense to encompass any of a range of vaporized orgaseous elements, compounds, or materials. Other terms used herein, suchas: reactant, precursor, vacuum, and inert gas, for example, all havetheir conventional meanings as would be well understood by those skilledin the materials deposition art.

The present invention is useful for making transistors with variabledielectric layer thickness. The ability to spatially control thedielectric layer thickness provides a means for adding extra protectionagainst shorting, reduced capacitance, and the use of dielectric layerthickness to size the various TFTs in a circuit, in addition to thecommonly used channel dimensions, width (W) and length (L). There aretwo types of TFTs useful in circuit design—enhancement-mode anddepletion-mode devices. An enhancement-mode transistor is a transistorin which there is negligible off-current flow relative to on-currentflow between a source and a drain at zero gate voltage. In other words,the transistor device is normally off. In contrast, a depletion-modetransistor is normally on meaning that more than substantiallynegligible current flows between a source and a drain at zero gatevoltage.

When the TFT operates in an enhancement-mode, the charges injected fromthe source electrode into the semiconductor are mobile and a currentflows from source to drain, mainly in a thin channel region within about100 Angstroms of the semiconductor-dielectric interface. See A.Dodabalapur, L. Torsi H. E. Katz, Science 1995, 268, 270, herebyincorporated by reference. In the absence of a gate field the channelideally has few charge carriers; as a result there is ideally nosource-drain conduction when the device is in off mode. The off-currentin an enhancement-mode device is defined as the current flowing betweenthe source electrode and the drain electrode when charge has not beenintentionally injected into the channel by the application of a gatevoltage. This occurs for a gate-source voltage more negative, assumingan re-channel device, than a certain voltage known as the thresholdvoltage. See Sze in Semiconductor Devices—Physics and Technology, JohnWiley & Sons (1981), pages 438-443, hereby incorporated by reference.The on-current is defined as the current flowing between the sourceelectrode and the drain electrode when charge carriers have beenaccumulated intentionally in the channel by application of anappropriate voltage to the gate electrode and the channel is conducting.For an n-channel accumulation-mode TFT, this occurs at a gate-sourcevoltage more positive than the threshold voltage. It is desirable forthis threshold voltage to be slightly positive, for enhancement moden-channel operation. Switching between on and off states is accomplishedby the application and removal of an electric field from the gateelectrode across the gate dielectric to the semiconductor-dielectricinterface, effectively charging a capacitor.

Attractive enhancement mode TFT device characteristics include a largeratio of the on-current to the off-current, and a steep sub-thresholdslope. In the operation of such a TFT device, a voltage applied betweenthe source and drain electrodes establishes a substantial current flowonly when the control gate electrode is energized. That is, the flow ofcurrent between the source and drain electrodes is modulated orcontrolled by the bias voltage applied to the gate electrode. Therelationship between material and device parameters of thezinc-oxide-based semiconductor TFT can be expressed by the approximateequation (see Sze in Semiconductor Devices—Physics and Technology, JohnWiley & Sons (1981)):

$I_{d} = {\frac{W}{2L}\mu \mspace{11mu} {C_{ox}\left( {V_{g} - V_{th}} \right)}^{2}}$

where I_(d) is the saturation source-drain current, C_(ox) is thegeometric gate capacitance associated with the insulating layer, W and Lare physical device dimensions, μ is the carrier mobility in thezinc-oxide-based semiconductor, V_(g) is the applied gate voltage, andV_(th) is the threshold voltage. Ideally, the TFT allows passage ofcurrent only when a gate voltage of appropriate polarity is applied.However, with zero gate voltage, the off current between source anddrain depends on the intrinsic conductivity a of the zinc-oxide-basedsemiconductor as well as the state of the semiconductor back channel.

The TFT structures described herein include an inorganic semiconductorchannel, preferably a metal-oxide-based semiconductor, with conductingelectrodes, commonly referred to as a source and a drain, for injectinga current into the semiconductor and a capacitively coupled gate forcontrolling and/or modulating the source-drain current. Preferred metaloxide based semiconductors include ZnO based semiconductors. Examples ofZnO based semiconductors are ZnO, InZnO, InO, and InGaZnO and doped ZnO.

The production of inorganic thin film transistors and electronic devicesfrom semiconductor, dielectric, conductive materials can be accomplishedby conventional techniques known to the skilled artisan. The exactprocess sequence is determined by the structure of the desiredtransistor. Thus, in the production of a field effect transistor in aso-called inverted structure, a gate electrode can be first deposited ona substrate, for example a vacuum- or solution-deposited metal ororganic conductor, or an ALD-deposited conductor. The gate electrode isinsulated with a dielectric and the source and drain electrodes and alayer of the inorganic semiconductor material are applied on top. Thestructure of such a transistor, and hence the sequence of itsproduction, are varied in the customary manner known to a person skilledin the art. Alternatively, a gate electrode is deposited first, followedby a gate dielectric, the semiconductor is applied, and finally thecontacts for the source electrode and drain electrode are deposited onthe semiconductor layer. In an alternative third structure, often calleda top-gate structure, the source and drain electrodes are depositedfirst, then the semiconductor is deposited, and finally the dielectricand gate electrode are deposited on top. In yet another embodiment, thesemiconductor is deposited first, prior to depositing the source anddrain electrodes. In most embodiments, a field effect transistorincludes an insulating layer, a gate electrode, a semiconductor layerincluding an inorganic material as described herein, a source electrode,and a drain electrode. The insulating layer, the gate electrode, thesemiconductor layer, the source electrode, and the drain electrode arein any sequence as long as the gate electrode and the semiconductorlayer contact opposite sides of the insulating layer and the sourceelectrode and the drain electrode both contact the semiconductor layer.

For conductive layers, such as those used for the gate, source, anddrain electrodes, useful materials include metals and conductive metaloxides. Examples of useful conductors include Al, Cr, Mo, Ag, aluminumdoped ZnO (AZO), and indium doped tin oxide (ITO). The gate dielectriccan be an inorganic gate dielectric, including for example, aninsulating metal oxide such as aluminum oxide, HfO, ZrO, or SiO2. Theinorganic semiconductor layer can be a metal oxide semiconductor,including for example, a ZnO based semiconductor. These layers of theTFT can be deposited using any method known in the art, including CVD,PECVD, sputtering or evaporation. ALD, and more specifically spatialALD, is a useful method of deposition due to the low required processingtemperature. The active layers can be patterned using methods including,selective area deposition (SAD), sequential masking, photolithography,laser, and/or other means known to the skilled artisan.

The embodiments of the present invention relate to thin film inorganicmaterials and devices that contain them. Example embodiments of thepresent invention use selective area deposition (SAD) in combinationwith atomic layer deposition (Am). SAD employs a patterned materialreferred to as a “deposition inhibitor material”, “deposition inhibitingmaterial”, or simply an “inhibitor” that inhibits the growth of a thinfilm material on the substrate when the substrate is subjected to anatomic layer deposition. Others have reported on using a selective areato generally pattern ALD coatings and more specifically to pattern asemiconductor layer. For example, Sinha et al. (J. Vac. Sci. Technol. B24 6 2523-2532 (2006)), have remarked that selective area ALD requiresthat designated areas of a surface be masked or “protected” to preventALD reactions in those selected areas, thus ensuring that the ALD filmnucleates and grows only on the desired unmasked regions. It is alsopossible to have SAD processes where the selected areas of the surfacearea are “activated” or surface modified in such a way that the film isdeposited only on the activated areas. There are many potentialadvantages to selective area deposition techniques, such as eliminatingan etch process for film patterning, reduction in the number of cleaningsteps required, and patterning of materials which are difficult to etch.The use of SAD to pattern multilayer dielectric layers for use inelectronic devices has been discussed in at least one of US PublicationNos. 20140065838, 20140065830, 20140061869, 20140061795, 20140061648 andU.S. Pat. Nos. 8,791,023; 8,846,545; and 8,653,516, the disclosures ofwhich are incorporated by reference herein.

By inhibiting the growth where the deposition material is present, thedeposition only deposits in regions (selective areas) of the substratewhere the inhibitor is not present. The phrase “deposition inhibitormaterial” and its equivalents refer herein to any material on thesubstrate that inhibits the deposition of material during atomic layerdeposition (ALD). The “deposition inhibitor material” includes thematerial applied to the substrate as well as the material resulting fromany optionally subsequent crosslinking or other reaction that modifiesthe material that may occur prior to depositing an inorganic thin filmon the substrate by atomic layer deposition. A polymeric depositioninhibitor material may be crosslinked after applying the polymer ontothe substrate, before or during the pattering step.

The deposition inhibitor material can be a compound or polymer that,after being applied, is subsequently polymerized, crosslinked, orpolymerized and crosslinked. The deposition inhibitor material can be acompound or polymer that forms a self-assembled monolayer on thesubstrate. Polymers are preferably addition polymers such as, forexample, a poly(perfluoroalkyl methacrylate); poly(perfluoroalkylmethacrylate); poly(methyl methacrylate); poly(cyclohexyl methacrylate);poly(benzyl methacrylate); poly(iso-butylene);poly(9,9-dioctylfluorenyl-2,7-diyl); polystyrene; poly(vinyl alcohol);poly(methyl methacrylate); poly(hexafluorobutyl methacrylate), andcopolymers thereof, wherein the alkyl has one to six carbon atoms.

Crosslinking can be used to insolubilize a polymeric depositioninhibitor material after application onto the surface of the substrate.The crosslinking can occur prior to patterning or may occur duringpatterning in order to contribute to the patterning step, for example,by employing crosslinking initiated by, and patterned by, actinicradiation, followed by removal of non-crosslinked polymer, for example,by solvent.

The deposition inhibiting material layer includes one of aself-assembled monolayer, a polymer, and a water-soluble polymer. Theself-assembled monolayer may be performed by exposing the substrate to avapor, a liquid, or a liquid solution of a precursor material. Thepolymer may be soluble in any convenient solvent and may have any usefulmolecular weight, preferably in the range of 2,000 to 2,000,000. It mayinclude a single functional group, or may include a plurality offunctional groups. In the case of a plurality of functional groups, thepolymer may be a random, periodic, or block polymer. Polymers soluble inpolar solvents such as water, alcohols, or ketones are particularlypreferred. Polymers may include amide groups, such as poly(amide),poly(vinylpyrollidone), and poly(2-ethyl-oxazoline). Polymers mayinclude ether linkages, such as poly(ethylene glycol). Polymers mayinclude alcohol functionalities, such as poly(vinyl alcohol). Polymersmay include neutralized acid groups such as sodium poly(styrenesulfonate) and the sodium salt of poly(acrylic acid).

In some embodiments, the deposition inhibitor material is chosenspecifically for the material to be deposited. The deposition inhibitormaterial has a given inhibition power. The inhibition power is definedas the layer thickness at or below which the deposition inhibitormaterial is effective. Preferably, the deposition inhibitor material,during use, exhibits an inhibition power of at least 50 Å, morepreferably at least 100 Å, most preferably at least 300 Å. Thedeposition of the deposition inhibitor material can be in a patternedmanner, such as using inkjet, flexography, gravure printing,microcontact printing, offset lithography, patch coating, screenprinting, or transfer from a donor sheet. In alternative embodiments, auniform layer of the deposition inhibitor material can be deposited andthen patterned form a patterned layer of the deposition inhibitormaterial. Preprocessing treatments for patterning the inhibitor includepatterning of substrate prior to inhibitor application to modify thehydrophobilicity, electric charge, absorption, or roughness of thesubstrate. Post processing treatments include light exposure, lightexposure and subsequent liquid based development, and ablation.

Providing the patterned deposition inhibiting material layer on thesubstrate includes using at least one of an inkjet printing process, aflexographic printing process, a gravure printing process, and aphotolithographic printing process. The active inhibiting material maybe suspended or dissolved in a solvent or vehicle. The material mayinclude surfactants, stabilizers, or viscosity modifiers. The printedmaterial may be dried using natural convection, forced convection, orradiant heat. The material may be treated to change its morphology orchemical composition. A preferred chemical composition change is tocrosslink the material. The change in morphology or chemical compositionmay be accomplished by exposure to a vapor phase or liquid phasereactant, or treatment with heat or light. Preferred processes includethe crosslinking of material with ultra-violet (UV) light.

The process of making the thin films of the present invention can becarried out below a maximum support temperature of about 300° C., morepreferably below 250° C., or even at temperatures near room temperature(about 25° C. to 70° C.). These temperatures are well below traditionalintegrated circuit and semiconductor processing temperatures andtherefore enable the use of any of a variety of relatively inexpensivesupports, such as flexible polymeric supports. Thus, the inventionenables production of relatively inexpensive circuits containing thinfilm transistors with significantly improved performance.

The substrates used in the present invention can be any material thatacts as a mechanical support for the subsequently coated layers. Thesubstrate can include a rigid material such as glass, silicon, ormetals. Particularly useful metals are stainless steel, steel, aluminum,nickel, and molybdenum. The substrate can also include a flexiblematerial such as a polymer film or paper such as Teslin. Usefulsubstrate materials include organic or inorganic materials. For example,the substrate can include inorganic glasses, ceramic foils, polymericmaterials, filled polymeric materials, coated metallic foils, acrylics,epoxies, polyamides, polycarbonates, polyimides, polyketones,poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(sometimes referred to as poly(ether ether ketone) or PEEK),polynorbomenes, polyphenyleneoxides, poly(ethylenenaphthalenedicarboxylate) (PEN), poly(ethylene terephthalate) (PET),poly(ether sulfone) (PES), poly(phenylene sulfide) (PPS), andfiber-reinforced plastics (FRP). The thickness of substrate 110 canvary, typically from about 100 μm to about 1 cm.

A flexible support or substrate can be used in the present invention.Using a flexible substrate allows for roll processing, which can becontinuous, providing economy of scale and economy of manufacturing overflat or rigid supports. The flexible support chosen is preferablycapable of wrapping around the circumference of a cylinder of less thanabout 50 cm in diameter, more preferably 25 cm in diameter, and mostpreferably 10 cm in diameter, without distorting or breaking, using lowforce as by unaided hands. The preferred flexible support can be rolledupon itself. Additional examples of flexible substrates include thinmetal foils such as stainless steel provided the foils are coated withan electrically insulating material layer to electrically isolate anyelectric components such as thin film transistors. Nominally rigidmaterials that are flexible due to their thinness may also be used.These include glass at thicknesses below 200 μm and metals atthicknesses below 500 μm.

In some example embodiments, the substrate can include a temporarysupport or support material layer, for example, when additionalstructural support is desired for a temporary purpose, e.g.,manufacturing, transport, testing, or storage. In these exampleembodiments, substrate can be detachably adhered or mechanically affixedto the temporary support. For example, a flexible polymeric support canbe temporarily adhered to a rigid glass support to provide addedstructural rigidity during the transistor manufacturing process. Theglass support can be removed from the flexible polymeric support aftercompletion of the manufacturing process.

The substrate can be bare indicating that it contains no substantialmaterials on its surface other the material from which it is composed.The substrate can include various layers on the surface. These layersinclude subbing layers, adhesion layers, release layers, wetting layers,hydrophilic layers, and hydrophobic layers. The substrate surface can betreated in order to promote various properties. These treatments includeplasma treatments, corona discharge treatments, and chemical treatments.

The substrate can also include on its surface patterned materials. Thesepatterns may include patterns that modulate light transmission orelectrical conductivity within or on the substrate. The patterns mayinclude complete devices, circuits, or active elements existing on thesubstrate. The patterns may include portions of devices, circuits, oractive elements awaiting subsequent processing steps for completion.

Atomic Layer Deposition (ALD) is a process which is used to producecoatings with thicknesses that can be considered consistent, uniform, oreven exact. ALD produces coatings that can be considered conformal oreven highly conformal material layers. Generally described, an ALDprocess accomplishes substrate coating by alternating between two ormore reactive materials commonly referred to as precursors, in a vacuumchamber. A first precursor is applied to react with the substrate. Theexcess of the first precursor is removed is removed from the vacuumchamber. A second precursor is then applied to react with the firstprecursor on the substrate. The excess of the second precursor isremoved from the vacuum chamber and the process is repeated.

Advantageously, ALD steps are self-terminating and can deposit preciselyone atomic layer when conducted up to or beyond self-terminationexposure times. An atomic layer typically ranges from about 0.1 to about0.5 molecular monolayers, with typical dimensions on the order of nomore than a few Angstroms. In ALD, deposition of an atomic layer is theoutcome of a chemical reaction between a reactive molecular precursorand the substrate. In each separate ALD reaction-deposition step, thenet reaction deposits the desired atomic layer and substantiallyeliminates “extra” atoms originally included in the molecular precursor.In its most pure form, ALD involves the adsorption and reaction of eachof the precursors in the complete absence of the other precursor orprecursors of the reaction. In practice, as in any process, it isdifficult to avoid some direct reaction of the different precursorsleading to a small amount of chemical vapor deposition reaction. Thegoal of any process claiming to perform ALD is to obtain deviceperformance and attributes commensurate with an ALD process whilerecognizing that a small amount of CVD reaction can be tolerated.

In ALD processes as described, typically two molecular precursors areintroduced into the ALD reactor in separate stages. Recently, a new ALDprocess has been developed which negates the need for a vacuum chamber.U.S. Patent Application Publication 2005/0084610 (Selitser) discloses anatmospheric pressure atomic layer chemical vapor deposition process thatinvolve separate chambers for each stage of the process and a series ofseparated injectors are spaced around a rotating circular substrateholder track. A spatially dependent ALD process, or spatial ALD (S-ALD),can be accomplished using one or more of the systems or methodsdescribed in more detail in at least one of U.S. Pat. No. 7,413,982,U.S. Pat. No. 7,456,429, U.S. Pat. No. 7,789,961, and US 2009/0130858,the disclosures of which are incorporated by reference herein. S-ALDproduces coatings with thicknesses that can be considered consistent,uniform, or even exact. S-ALD produces coatings that can be consideredconformal or even highly conformal material layers. S-ALD is alsocompatible with a low temperature coating environment. Additionally,S-ALD is compatible with web coating, making it attractive for largescale production operations. Even though some web coating operations mayexperience alignment issues, for example, web tracking or stretchingissues, the architecture of the present invention reduces reliance onhigh resolution or very fine alignment features during the manufacturingprocess. As such, S-ALD is well suited for manufacturing the presentinvention.

The preferred process of the present invention employs a continuousspatially dependent ALD (as opposed to pulsed or time dependent ALD)gaseous material distribution. The process of the present inventionallows operation at atmospheric or near-atmospheric pressures and iscapable of operating in an unsealed or open-air environment. The processof the present invention is adapted such that material is deposited onlyin selected areas of a substrate.

Atomic layer deposition can be used in the present invention to deposita variety of inorganic thin films that are metals or that comprise ametal-containing compound. Such metal-containing compounds include, forexample (with respect to the Periodic Table) a Group V or Group VIanion. Such metal-containing compounds can, for example, include oxides,nitrides, sulfides or phosphides of zinc, aluminum, titanium, hafnium,zirconium or indium, or combinations thereof.

Oxides that can be made using the process of the present inventioninclude, but are not limited to: zinc oxide (ZnO), aluminum oxide(Al₂O₃), hafnium oxide, zirconium oxide, indium oxide, tin oxide, andthe like. Mixed structure oxides that can be made using the process ofthe present invention can include, for example, InZnO. Doped materialsthat can be made using the process of the present invention can include,for example, ZnO:Al, Mg_(x)Zn_(1-x)O, and LiZnO.

A dielectric material is any material that is a poor conductor ofelectricity. Such materials typically exhibit a bulk resistivity greaterthan 10¹⁰ Ω-cm. Examples of dielectrics are SiO₂, HfO, ZrO, SiNx, andAl₂O₃. A semiconductor is a material in which electrical charges canmove but in which the concentration of electrical charges can besubstantially modulated by external factors such as electrical fields,temperature, or injection of electrical charges from a neighboringmaterial. Examples of semiconductors include silicon, germanium, andgallium arsenide. Particularly preferred semiconductors are zinc oxide,indium zinc oxide, and gallium indium zinc oxide. The semiconductors maybe doped to render them n-type or p-type, or to modulated the number ofcharge carriers present.

Metals that can be made using the process of the present inventioninclude, but are not limited to: copper, tungsten, aluminum, nickel,ruthenium, and rhodium. It will be apparent to the skilled artisan thatalloys of two, three, or more metals may be deposited, compounds may bedeposited with two, three, or more constituents, and such things asgraded films and nano-laminates may be produced as well.

These variations are simply variants using particular embodiments of theinvention in alternating cycles. There are many other variations withinthe scope of the invention, so the invention is limited only by theclaims that follow.

For various volatile zinc-containing precursors, precursor combinations,and reactants useful in ALD thin film processes, reference is made tothe Handbook of Thin Film Process Technology, Vol. 1, edited by Glockerand Shah, Institute of Physics (IOP) Publishing, Philadelphia 1995,pages B1.5:1 to B1.5:16, hereby incorporated by reference, and Handbookof Thin Film Materials, edited by Nalwa, Vol. 1, pages 103 to 159,hereby incorporated by reference, including Table V1.5.1 of the formerreference.

Although oxide substrates provide groups for ALD deposition, plasticsubstrates can be used by suitable surface treatment.

In a preferred embodiment, ALD can be performed at or near atmosphericpressure and over a broad range of ambient and substrate temperatures,preferably at a temperature of under 300° C. Preferably, a relativelyclean environment is needed to minimize the likelihood of contamination;however, full “clean room” conditions or an inert gas-filled enclosurewould not be required for obtaining acceptable performance when usingpreferred embodiments of the process of the present invention.Preferably, the present process can be accomplished using a spatial ALDprocess.

Two suitable approaches to combining patterning and depositing thesemiconductor are described in US Patent Application Publication No.2009/0081827 A1, published to Yang et al., on Mar. 26, 2009, thedisclosure of which is hereby incorporated by reference in its entirety;and U.S. Pat. No. 8,017,183 B2, issued to Yang et al., on Sep. 13, 2011,the disclosure of which is hereby incorporated by reference in itsentirety. Given that the preferred subsequent layers are deposited andconformally coated by atomic layer deposition (ALD), preferreddeposition inhibitor materials are described in U.S. Pat. No. 7,998,878B2, issued to Levy et al., on Aug. 16, 2011, the disclosure of which ishereby incorporated by reference in its entirety. In addition,deposition inhibitor materials are chosen to be compatible with printingon large flexible substrates in a low cost manner.

In some embodiments of the present invention, treating the surface priorto depositing a layer by ALD is an important process step. For thedescription that follows, the term “treating” can be taken to meansubjecting the substrate to a different environmental condition thanthat experienced during the ALD deposition. Treating can occur eitherwithin the ALD system, or preferably, the substrate is removed from thesystem and treated external to the system. Surface treatments includedry cleaning, such as a UV-ozone treatment, or a plasma, for example andpreferably an oxygen plasma. Other treatments include wet clean steps,hold at ambient conditions, wet or dry etching the surface of a materiallayer, and other processes. An example cleaning process includes aliquid process using a solvent or a detergent. The liquid process mayutilize a mechanical action such as brushing or wiping or pressure jets.The cleaning process can also be a vapor process. These processesinclude exposing the substrate to a vapor reactant that causes cleaning.The vapor exposure may include forms of energy to promote the process.These include light exposure, arcs, or plasmas. Particularly desiredlight exposures include UV exposure, especially in the presence ofoxygen to produce ozone (UV-ozone). Plasmas include plasmas of variousspecies including oxygen, chlorine, and fluorine. Plasmas created withthese materials or with precursors that produce these materials areincluded in the present invention.

Turning now to the figures, FIG. 1a is a cross-sectional diagram of anembodiment of a TFT 100 of the present invention, taken along the lineA-A′ of the plan view shown in FIG. 1b . The TFT 100 shown in FIGS. 1aand 1b is a bottom gate structure having one embodiment of the variablethickness dielectric layer of the present invention. In this embodiment,TFT 100 has a gate 120 in contact with a substrate 110, a variablethickness dielectric stack 150 that is in contact with the gate 120, thesubstrate 110, and a semiconductor layer 170. The semiconductor layer170 is in contact with a source electrode 180 and a drain electrode 185.TFT 100 shown in FIGS. 1a and 1b is a bottom gate structure, but itshould be understood that transistors with a top gate structure composedof the same layers can be constructed and are considered an alternativeembodiment of the present invention. A top gate TFT of the presentinvention has source and drain electrodes 180, 185 in contact with thesubstrate 110, a semiconductor layer 170 that is in contact with thesource/drain 180,185 and the substrate 110, and a variable thicknessdielectric stack 150 in contact with the semiconductor layer 170 and thegate 120.

As also shown in FIG. 1b , on substrate 110 there is a gate 120including a first electrically conductive layer. The substrate 110 canbe any previously discussed substrate, and may contain a plurality ofpredefined layers. The gate 120 has the conventionally accepted meaning,and is used to gate the current of the TFT 100. The first electricallyconductive layer of the gate 120 may be a single conductive material, asshown in FIG. 1a , or may comprise any number of conductive materiallayers.

In order to keep the gate 120 isolated from the source and drainelectrodes 180,185, a variable thickness dielectric stack 150 isdisposed between them. Variable thickness dielectric stack 150 is madeup of patterned first inorganic thin film dielectric material layer 130and patterned second inorganic thin film dielectric material layer 140.The first inorganic thin film dielectric layer 130 has a first patternand the second inorganic thin film dielectric layer 140 has a differentsecond pattern. As shown in FIGS. 1a and 1b , the first pattern of thefirst dielectric layer 130 includes a portion having a via 814 over thegate in the channel region of the device (the gate via 814 indicatesthat layer 130 has no appreciable thickness), and portions between thesource and drain electrodes 180,185 and the gate electrode 120 having athickness. The second inorganic thin film dielectric layer 140 has asecond pattern that is different from that of the first pattern, and hasa uniform thickness over the gate 120, both in the area between thesource and drain electrodes 180, 185 and the gate, and within thechannel region of the TFT 100.

The combination of the patterns of first and second thin film dielectriclayers 130, 140 form a dielectric stack 150 in contact with the gate 120having first, second, and third regions, the first region in contactwith the semiconductor layer 170 in the channel region and having afirst thickness, the second region adjacent to the first region havingthe first thickness, and the third region adjacent to the second regionhaving a second thickness greater than the first thickness. Plainlystated, the dielectric stack 150 provides a gate dielectric thicknessthat is thinner than the dielectric thickness in much of the overlapregion between the source and drain electrodes 180,185 and the gate 120.Additionally, the size of the thinnest portion of the dielectric stack150 located in the channel region can be independently sized from theother portions of the dielectric stack 150. It is preferable that, asshown, there is a second region adjacent to the channel with the samethickness as the gate dielectric portion to ensure that the source anddrain electrodes 180, 185 contact the semiconductor layer 170 in thethinner region of dielectric thickness. This extension of the gatedielectric thickness region beyond the channel region defined by the gapbetween the source and drain electrodes 180, 185 provides for goodcarrier injection and for robustness for alignment in fabrication of theTFT 100 shown in FIGS. 1a and 1 b.

In alternative embodiments, the first patterned inorganic thin filmdielectric layer 130 can be formed in contact with the semiconductorlayer 170, and the second patterned inorganic thin film dielectric layer140 can be formed in contact with the substrate 110. This reversal oforder of the first and second thin film dielectric layers 130, 140results in an equivalent variable thickness dielectric stack 150. Thefirst and second thin film dielectric layers 130, 140 can have differentmaterial compositions, or in a preferred embodiment the same materialcomposition.

In embodiments where the patterned first inorganic thin film dielectricmaterial 130 and patterned second inorganic thin film dielectricmaterial 140 have the same material composition, the variable thicknessdielectric stack 150 will not have the same analytical signature as asingle layer with a combined thickness of the same material in thesecond region where the thickness of each layer is present (secondthickness). The analytical signature is a consequence of forming thepatterned first inorganic thin film dielectric material 130 in aseparate step, and after at least a temporary change in environment,than the patterned second inorganic thin film dielectric material 140,resulting in an interface region between the first and second patternedinorganic dielectric layers 130 and 140. There will be a change in theintensity signal of either an impurity or compositional species in theinterface region when compared to a region of the first inorganic thinfilm dielectric material layer 130 and the second inorganic thin filmdielectric material layer 140 that is outside of the interface region.This intensity change acts as a marker indicating that an inorganicdielectric layer is indeed a variable thickness dielectric stack 150 andnot a single layer of material that has been selectively etched to havedifferent thicknesses. When an intensity signal for an impurity orcompositional species in a contact area between the first inorganic thinfilm dielectric layer 130 and the second inorganic thin film dielectriclayer 140 is 50% or greater than the intensity signal for the impurityor compositional species in a region of the first inorganic thin filmdielectric layer 130 and the second inorganic thin film dielectric layer140 that is outside of the contact region is detected, it serves as amarker indicating that an inorganic dielectric layer is not a singlelayer of material. The first inorganic thin film dielectric layer 130and the second inorganic thin film dielectric layer each have anassociated thickness. As shown in FIG. 1a , the thickness of the firstinorganic thin film dielectric layer 130 and the thickness of the secondinorganic thin film dielectric layer 140 are same. In alternativeembodiments of the present inventions, these film thickness may bedifferent. Additionally, one or both of the first and second thin filmdielectric layers 130,140 can be formed of one or more dielectriclayers.

FIGS. 1a and 1b show the relative location of the source and drainelectrodes 180,185 on substrate 110. The source and drain have theconventionally accepted meaning, and either electrode shown may bedesignated the source (or drain) as is required by the application orcircuit. The source and drain electrodes 180,185 are in a secondelectrically conductive layer. As with the first electrically conductivelayer, the second electrically conductive layer can be a singleconductive material, as shown in FIG. 1a , or alternatively can compriseany number of conductive material layers.

An advantage of the variable thickness dielectric layer 150 is that thesize of the gate via 814 can be sized independently of the size andlocation of the other elements in the TFT 100. In prior art processesfor forming transistors with variable thickness dielectric layers, theregion of extra dielectric thickness between the overlap of the gate andsource/drain layers is aligned, and most often self-aligned, with thesource and drain. This use of an aligned structure results in thedielectric thickness immediately adjacent the channel region beingthicker than the gate dielectric thickness in the channel. As aconsequence, prior art uses of extra dielectric thickness in the overlapbetween the gate and the source/drain have been limited to top gatearchitectures. This limitation is a functional limitation; if aself-aligned extra dielectric portion was used in a bottom gatearchitecture, it would prevent good charge injection into thesemiconductor since the region of contact of the source and drain withthe semiconductor would be separated from the gate by a dielectrichaving a thickness greater than the gate dielectric thickness. Anotheradvantage of the present invention is that the variable thicknessdielectric layer 150 can be implemented in any TFT architecture bottomgate or top gate, with or without staggered contacts.

FIG. 2 is a cross-sectional diagram of an alternative embodiment of aTFT 102 of the present invention, taken along the line A-A′ of the planview shown in FIG. 1b . The TFT 102 shown in FIG. 2 is a bottom gatestructure similar to TFT 100 shown in FIG. 1a , and also has a gate 120in contact with the substrate 110 and a variable thickness dielectricstack 152. The variable thickness dielectric stack 152 is in contactwith the gate 120, the substrate 110, and the semiconductor layer 170.The semiconductor layer 170 is in contact with the source electrode 180and the drain electrode 185. The substrate 110, gate 120, semiconductorlayer 170, and source and drain electrode 180,185 should be understoodfrom the previous description of FIGS. 1a and 1 b.

The variable thickness dielectric stack 152 shown in FIG. 2 serves thesame function and has the same defining features as the variablethickness dielectric stack 150 shown in FIG. 1a . The dielectric stack152 is in contact with the gate 120 and has first, second, and thirdregions, where the first region is in contact with the semiconductorlayer 170 in the channel region and has a first thickness, the secondregion is adjacent to the first region and also has the first thickness,and the third region that is adjacent to the second region having asecond thickness greater than the first thickness. As discussed withrespect to FIG. 1a , this feature of variable thickness provides athicker dielectric where the source and drain electrodes 180, 185overlap the much of gate 120, including the gate edge, reducing thepotential for shorting and decreasing the gate leakage. The thinnerdielectric over the gate 120 in the channel region of the devicecontrols the electrical field experienced in the semiconductor layer 170when the gate 120 is activated. The additional dielectric buffer layer160 in dielectric stack 152 serves to control the interface between thesemiconductor layer 170 and the gate dielectric 152, which is known tobe critical to the function of the TFT 102. In TFTs that are formed bythe combination of SAD and ALD, special care should be taken to insurethat the interface between the semiconductor and the gate dielectric isnot disturbed by the removal of the deposition inhibiting material. Asshown in FIG. 2, the dielectric buffer layer 160 has the same pattern asthe semiconductor layer 170 and is in contact with the semiconductorlayer 170. The additional dielectric buffer layer 160 can be a differentmaterial from the other layers in the variable thickness dielectricstack 152. Preferably, all of the layers of the variable thicknessdielectric stack 152, including the additional dielectric buffer layer160, have the same material composition. As discussed above, althougheach layer is formed of the same material, when the layers are formed inseparate steps the interfaces between the layers can be detected by achange in the intensity signal of either an impurity or compositionalspecies.

Variable thickness dielectric stack 152 is made up of patterned firstinorganic dielectric layer 130, patterned second dielectric layer 140,and the additional dielectric buffer layer 160. The first inorganic thinfilm dielectric layer 130 has a first pattern and the second inorganicthin film dielectric layer 140 has a second pattern. As shown in FIGS.1a and 1b , the first pattern of the first dielectric layer 130 includesa portion having a via 814 over the gate 120 in the channel region ofthe device (the via indicates that layer 130 has no appreciablethickness) and other portions of the pattern between the source anddrain electrodes 180,185 and the gate electrode 120 have a largerthickness. The second inorganic thin film dielectric layer 140 has apattern that is different from that of the first pattern, and has auniform thickness over the gate, both in the area where the source anddrain electrodes 180,185 overlap the gate 120 and within the channelregion of the TFT 102. The additional dielectric buffer layer 160 has auniform thickness in the area of the semiconductor pattern. Plainlystated, the variable thickness dielectric stack 152 provides a gatedielectric thickness that is thinner than the dielectric thickness inmuch of the overlap region between the source and drain electrodes180,185 and the gate 120. Additionally, the thinnest portion of thedielectric stack in the channel region can be independently sized fromthe other portions of the dielectric stack 152. It is preferable that,as shown, there is a second region adjacent to the channel having thesame thickness as the gate dielectric within the channel region toensure that that the source and drain electrodes 180,185 contact thesemiconductor layer 170 in a region of proper dielectric thickness; asshown in FIG. 2 this thickness is equal to the combined thickness of thesecond dielectric layer 140 and the additional dielectric buffer layer160. This extension of the gate dielectric thickness region beyond thechannel region defined by the gap between the source and drainelectrodes 180,185 enables good charge injection and also robustalignment when fabricating the TFT 102 shown in FIGS. 2 and 1 b. Inalternative embodiments, the first patterned dielectric layer 130 isformed in contact with the additional dielectric buffer layer 160, andthe second patterned inorganic thin film dielectric layer 140 is formedin contact with the substrate 110. This reversal of order of the firstand second thin film dielectric layer results 130, 140 in an equivalentvariable thickness dielectric stack 152.

FIG. 3 is a cross-sectional diagram of an alternative embodiment of aTFT 103 of the present invention, taken along the line A-A′ of the planview shown in FIG. 1b . The TFT 103 shown in FIG. 3 is a bottom gatestructure similar to TFT 100 shown in FIG. 1a , and also has a gate 120in contact with the substrate 110, and has a variable thicknessdielectric stack 153. The variable thickness dielectric stack 153 is incontact with the gate 120, the substrate 110, and the semiconductorlayer 170. The semiconductor layer 170 is in contact with the sourceelectrode 180 and the drain electrode 185. As shown in FIG. 3, thesubstrate 110, gate 120, semiconductor layer 170, and source and drainelectrode 180,185 should be understood from the previous description ofFIG. 1a and 1 b.

The TFT 103 of FIG. 3 illustrates an embodiment of the present inventionwherein the interface between the dielectric layer 153 and thesemiconductor layer 170 is controlled by dielectric buffer layer 143,and the thickness of the dielectric buffer layer 143 is the same as thethickness of the gate dielectric in the channel region. The firstinorganic thin film dielectric layer 130 has a first pattern and thedielectric buffer layer 143 has a second pattern. In the embodiment ofTFT 103, the dielectric buffer layer 143 is a second patterneddielectric layer. The first pattern of the first dielectric layer 130includes a portion over the gate in the channel region of the devicewith no thickness (i.e. gate via 814), and other portions of the patternbetween the source and drain electrodes 180,185 and the gate electrode120 having a larger thickness. The dielectric buffer layer 143 has asecond pattern that is different from that of the first pattern and hasa uniform thickness in the area between the source and drain electrodes180, 185 and the gate 120 within the channel region of the TFT 103. Asshown in FIG. 3, the second pattern is the same as the pattern of thesemiconductor layer 170. The combination of the patterns of first andsecond thin film dielectric layers 130, 143 form a dielectric stack 153in contact with the gate 120 having first, second, and third regions,the first region in contact with the semiconductor layer 170 in thechannel region and having a first thickness, the second region adjacentto the first region having the first thickness, and the third regionadjacent to the second region having a second thickness greater than thefirst thickness. Plainly stated, the dielectric stack 153 provides agate dielectric thickness in the channel region that is thinner than thedielectric thickness in much of the overlap region between the sourceand drain electrodes 180,185 and the gate 120.

In the embodiment shown in FIG. 3, the dielectric buffer layer 143 is incontact with both the gate 120 and the semiconductor layer 170. It isuseful to independently set the thickness of the gate dielectric fromthe portions of the variable thickness dielectric stack 153. It ispreferable that, as shown, there is a second region adjacent to thechannel with the same thickness as the gate dielectric portion to ensurethat that the source and drain electrodes 180, 185 contact thesemiconductor layer 170 in a region of proper dielectric thickness. Thisextension of the gate dielectric thickness region beyond the channelregion defined by the gap between the source and drain electrodes 180,185 enables good charge injection and also robust alignment duringfabrication of the TFT 103.

FIGS. 4a, 4b, and 4c serve to more clearly illustrate the criticalfeatures of the variable thickness dielectric stack 150 and the regionsreferred to above. The region of the transistor that lies over the gate120 along a direction parallel to the length of the transistor is shownin plan view in FIGS. 4a and 4b and in cross section in FIG. 4c . Theplan views of FIGS. 4a and 4b extend to the length of the gate. Asshown, the transistor has a gate 120, the gate 120 having a gate length425. The source and drain electrodes 180,185 are separated by a gapdefining a channel region, and the channel region has a channel length455 that is less than the gate length 425. The semiconductor layer 170is in contact with the source and drain electrodes 180, 185. As shown inFIG. 4b , the semiconductor layer 170 has a length 475 greater than thegate length 425 and extends beyond the gate 120. This feature of thesemiconductor layer 170 is useful to provide alignment tolerance in thefabrication process and further protects the source and drain electrodes180,185 from shorting to the gate 120.

There are three distinct regions of the dielectric stack illustrated inFIGS. 4b and 4c , the channel region 450, a first region 460 adjacent tothe channel region 460, and a second region 470 adjacent to the firstregion 460 and opposite the channel region 450. The channel region 450is within the channel of the TFT 100 and the thickness of the variablethickness dielectric layer is the gate dielectric thickness which willdetermine the performance of the TFT 100 by defining the capacitanceC_(ox) of Equation (1). The first region 460 includes the area where thesource and drain electrodes 180, 185 make contact to the semiconductorlayer 170 adjacent to the channel region 450. The dielectric thicknessin the first region 460 is the same as the thickness in the channelregion 450. Additionally, the dielectric in the first region 460 and thedielectric in the channel region 450 are formed from the same dielectriclayer or layers, and as such have the same composition. This extensionof the gate dielectric thickness beyond the channel region 450 providesmanufacturing robustness for TFT 100, and also is beneficial to deviceperformance by ensuring good contact and charge injection. As shown inFIGS. 4b and 4c , in some embodiments the dielectric in the first region460 and the dielectric in the channel region 450 are defined by via 814in the first patterned dielectric layer 130, and the length 465 of via814 is greater than the channel length 455 and less than the length 425of the gate 120. Farther away from the channel region 450, thedielectric in the second region 470 is thicker than that of thedielectric in the channel region 450. The thickness in the second region470 is equal to the maximum thickness of the variable thicknessdielectric stack 150, which is equal to the sum of the thicknesses ofall of the layers of the variable thickness dielectric stack 150. Thesecond region 470 is over a portion of the gate 120, and providesprotection against shorting of the source and drain electrodes 180,185to the gate 120. The additional dielectric thickness in the secondregion 470 also reduces the parasitic capacitance of the TFT 100.Furthermore, the second region 470 can reduce the gate leakage. Thevariable thickness dielectric stack 150 can be made up of at least twolayers, and as many additional dielectric layers as desired. In someembodiments, as shown in FIG. 2, the variable thickness dielectric stack150 has three layers. Preferably the variable thickness dielectric stack150 has fewer than ten layers. The variable thickness dielectric stack150 can have other regions outside of the area of the TFT 100 withthickness that are different from that of the first region 460 or thesecond region 470. The variable thickness dielectric stack 150 can beshared between multiple devices over a single substrate. Thedescriptions of variable thickness dielectric stack 150 should also aidin the understanding of the alternative embodiments 152, 153 shown inFIGS. 2 and 3.

Turning now to FIG. 5, a diagrammatic Step diagram is shown for oneembodiment of a process of the present invention for making a variablethickness dielectric stack using a combination of selected areadeposition (SAD) and ALD. The process for making a variable thicknessdielectric stack includes the steps of providing a substrate; forming afirst patterned conductive layer on the substrate; forming a firstpatterned deposition inhibitor layer having a first pattern over theconductive layer and the substrate; depositing a first dielectric thinfilm using ALD to form a first patterned conformal dielectric layerhaving the first pattern; removing the first patterned depositioninhibitor layer; forming a second patterned deposition inhibitor layerhaving a second pattern over the conductive layer and the substrate;depositing a second dielectric thin film using ALD to form a secondpatterned conformal dielectric layer having the second pattern; andforming a second patterned conductive layer. At least a portion of thefirst patterned conductive layer and a portion of the second patternedconductive layer overlap each other forming an overlap region. A portionof the first pattern or a portion of the second pattern extending intothe overlap region such that a first portion of the overlap regionincludes the first and the second dielectric thin films and a secondportion of the overlap region includes only one of the first or seconddielectric thin films.

Each Step is now examined in more detail, and may be better understoodby reference to the illustrative descriptive build shown incross-sectional views in FIGS. 6a-6i . In Step 1 of FIG. 5, asillustrated in FIG. 6a , a substrate is supplied into the system. Aspreviously described, the substrate can be any substrate that issuitable for the process and can contain any number of preexistinglayers. In Step 10 of FIG. 5, as illustrated in FIG. 6b a firstpatterned conductive layer is formed on the substrate. The firstpatterned conductive layer can be formed from any practicallyprocessable conductive material, including metals and conductive metaloxides. The first patterned conductive layer can be formed using anymethod known in the art, including photolithography and selective areadeposition. In a preferred embodiment, the first patterned conductivelayer is a patterned transparent conductive oxide (TCO). In otherpreferred embodiments, forming the first patterned conductive layerincludes using selective area deposition in combination with spatialALD.

In Step 15 of FIG. 5, as illustrated in FIG. 6c , a patterned layer ofdeposition inhibitor having a first pattern is formed over theconductive layer and the substrate. The deposition inhibitor materialcan generically be any material that causes the material deposition tobe inhibited and should be understood from the previous descriptions. Inone embodiment, the deposition inhibitor material is chosen specificallyfor the material to be deposited. In other embodiments, the depositioninhibitor material has a given inhibition power. The inhibition power isdefined as the layer thickness at or below which the depositioninhibitor material is effective. In some embodiments, the patternedlayer of deposition inhibitor is formed in Step 15 using an additivepatterning method, such as using inkjet, flexography, gravure printing,micro-contact printing, offset lithography, patch coating, screenprinting, or transfer from a donor sheet. A preferred inhibitor foradditive patterning methods is polyvinyl pyrrolidone (PVP). In analternative embodiment, patterned layer of deposition inhibitor isformed by coating a uniform layer of the deposition inhibitor materialpatterning using photolithographic techniques. A preferred inhibitor forphotolithographic patterning is polymethylmethacrylate (PMMA). As shownin FIG. 6c , the first pattern of the first pattern deposition inhibitorlayer 605 contains regions where deposition inhibitor is not present. Asshown FIG. 6c , the first pattern has a portion of pattern that overlapsthe first patterned conductive layer 620.

Continuing with FIG. 5, Step 20, as illustrated in FIG. 6d , depositsthe desired first dielectric layer 630 by an Atomic Layer Deposition(ALD) process. This deposition can be using any ALD system, preferably aspatial ALD system. The dielectric layer is deposited only in the areasof the substrate where there is no deposition inhibitor material, andforms a patterned conformal dielectric layer having the first pattern.As used herein, patterned layers formed using SAD are said to have thesame pattern as the layer of deposition inhibitor. This usage indicatesthat the patterns have the same x-y features, and it should beunderstood that the inorganic layer is formed in the opposite portion ofthe pattern than where the inhibitor is present. After the firstpatterned dielectric layer 630 is deposited in Step 20, the depositioninhibitor 605 is removed in Step 25 of FIG. 5, as illustrated in FIG. 6e, by a surface treatment that simultaneously cleans the surface of thefirst patterned dielectric layer. Surface treatments include drycleaning, such as a UV-ozone treatment, or gas plasma, preferably anoxygen plasma. Other treatments include wet clean steps.

A second patterned layer of deposition inhibitor 615 having a secondpattern is formed over the conductive layer and the substrate in Step 35of FIG. 5, as illustrated in FIG. 6f . As in Step 15, the depositioninhibitor material 615 may be deposited in a patterned manner using aprinting technique, or as a uniform layer that is patterned usingphotolithography. At least the first pattern or the second pattern has aregion over the first conductive layer where there is depositioninhibitor present. As illustrated in FIG. 6f , the second patterneddeposition inhibitor layer 615 has a second pattern that is differentfrom the first pattern of the first patterned deposition inhibitormaterial layer 605. The second patterned deposition inhibitor layer 615has regions where the inhibitor material is not present. In preferredembodiments, the first patterned layer of deposition inhibitor and thesecond patterned layer of deposition inhibitor are formed using the sameprocess with the same inhibitor material.

After providing the second patterned deposition inhibitor material layer615 on the substrate, a second dielectric layer 640 is deposited by ALDin Step 40 of FIG. 5, as illustrated in FIG. 6g . The second thin filmdielectric material 640 may be the different from that of the first thinfilm dielectric material 630, or preferably the same as that of thefirst thin film dielectric material. The layer thickness of the firstand second thin film dielectric material may be the same or different.After depositing the second thin film dielectric material, thedeposition inhibitor material 615 is removed in Step 50 of FIG. 5, asillustrated in FIG. 6h , using any of the processes discussed withrespect to Step 25. The patterned first inorganic dielectric layer 630and patterned second inorganic dielectric layer 640 in combination formthe variable thickness dielectric stack 650 as illustrated in FIG. 6hafter the inhibitor was removed in Step 50. The Steps 15 through 50shown in FIG. 5 can be considered as a single Step 60, which is aprocess block to form a variable thickness dielectric stack.

In Step 70, after removing the deposition inhibitor 615, a secondpatterned conductive layer 680 is formed over at least part of the firstconductive layer 620 and the variable thickness dielectric stack 650, asillustrated in FIG. 6i . The second patterned conductive layer 680 has aportion that overlaps at least a portion of the first patternedconductive layer 620 to form an overlap region. A portion of the firstpattern or a portion of the second pattern extends into the overlapregion such that there are at least two portions of the overlap region:a first portion of the overlap region that includes both the first andthe second dielectric layers, and a second portion of the overlap regionthat includes only one of the first or second dielectric layers. Asillustrated in FIG. 6i , the variable thickness dielectric stack 650 isdisposed between the first patterned conductive layer 620 and the secondpatterned conductive layer 680. The first patterned conductive layer 620and the second patterned conductive layer 680 overlap to form an overlapregion 675. The overlap region has two portions 690 and 695. The firstportion 690 of the overlap region 675 includes the thickness of both ofthe first and the second dielectric layers 630, 640, while the secondportion 695 of the overlap region 675 includes only the thickness of thesecond patterned dielectric layer 640 as shown in FIG. 6i . The twoportions 690, 695 of the variable thickness dielectric stack 650 withinthe overlap region are a result of the differences in the first andsecond patterns of the deposition inhibitor. Although as illustrated inFIGS. 6a-6i , the second portion 695 includes only the second dielectriclayer 640, alternative embodiments having only the first dielectriclayer 630 present in the second portion 695 are easily obtained byselecting the first and second patterns appropriately.

The process flow described in FIG. 5 is useful to make any number ofelectronic and optoelectronic elements. For example, a capacitor is alogical result of the process in FIG. 5 having a dielectric stackbetween two conductors. A transistor results from the addition of asemiconductor layer between the two conductive layers, and appropriatepatterning of the two conductors.

FIG. 7 is a variation of the diagrammatic Step diagram shown in FIG. 5,illustrating one embodiment of a process of the present invention formaking a variable thickness dielectric stack as part of a thin filmtransistor. Step 1 provides a substrate. In Step 710 a patterned gate isformed on the substrate. This is a more specific embodiment of FIG. 5that forms the first patterned conductive layer in Step 10.

Step 60, forming a variable thickness dielectric stack, as defined bySteps 15 through 50, is the same for FIGS. 5 and 7 and should beunderstood from the previous descriptions. After forming the variablethickness dielectric stack a patterned semiconductor layer is formedover the gate in Step 765. The variable thickness dielectric stack isdisposed between the semiconductor layer and the gate. After forming thesemiconductor layer, the source and drain electrodes are formed in Step770. Step 770 is a more specific embodiment of step 70 of FIG. 5, namelyforming the second conductive layer. The source and drain electrodes areseparated by a gap which defines a channel region. As shown in FIG. 7,Step 765 forming a patterned semiconductor layer over the gate, placesthe semiconductor patterned layer between the first patterned conductivelayer and the second patterned conductive layer, i.e. between the gateand the source and drain electrodes.

As with the first and second conductive layers of FIG. 5 and FIG. 6, thesource and the drain electrode each have a portion that overlaps atleast a portion of the gate to form an overlap region; this overlapregion is adjacent to the channel region. There are at least twoportions of the overlap region: a first portion of the overlap regionthat includes both the first and the second dielectric layers, and asecond portion of the overlap region which includes only one of thefirst or second dielectric layers. In the process of FIG. 7, the secondportion of the overlap region having only one of the first or seconddielectric layers is adjacent to the channel region, and this secondportion of the overlap region is between the channel region and thefirst portion of the overlap region. The channel region and the secondportion of the overlap region have the same thickness. The first portionof the overlap region is further from the channel including both thefirst and the second dielectric layers, and is useful to prevent againstthe source or drain electrode shorting to the gate.

FIG. 7 uses the process flow of FIG. 5 to describe forming a staggeredbottom gate structure where the gate is in contact with the substrate,and the semiconductor layer is under the source and drain electrodes. Itshould be clear that the process flow of FIG. 5, with the additionalstep of forming a patterned semiconductor layer, can be used for anythin film transistor structure including a non-staggered bottom gatestructure where the gate is in contact with the substrate and thesemiconductor is over the source and drain electrodes. The process isalso useful for forming top gate structures, where the source and drainelectrodes are in contact with the substrate and the semiconductor. Topgate architectures with the semiconductor layer over or under the sourceand drain electrodes are equally easy to obtain using the processdescribed herein.

FIGS. 8a and 8b through 23a and 23b are descriptive process buildfigures for the process flow shown in FIG. 7 to form a bottom gate TFThaving a variable thickness dielectric stack. It should be understoodfrom the previous descriptions that other architectures with thevariable dielectric stack are also obtainable using this method bychanging the order of forming the patterned layers.

FIG. 8a is a cross-sectional diagram for processing Step 10 to form oneembodiment of a TFT 100 of the present invention, taken along the lineA-A′ of the plan view shown in FIG. 8b . As shown in FIGS. 8a and 8b ,an electrically conductive gate 120 is formed on a substrate 110. In theExample TFTs of the present invention, the gate 120 was provided usingthe combination of SAD and ALD forming a gate having 1000 A of AZO asthe conductive layer. The gate 120 can be provided using any methodknown in the art, including using a selective area deposition process.In other embodiments of Step 10, a conductive material can be depositeduniformly and patterned using standard photolithographic techniques. Inother embodiments the gate 120 can be formed by directly printing aconductor.

A first patterned deposition inhibitor layer 810 is provided onsubstrate 110 as shown in FIGS. 9a and 9b , and contains regions 820where the deposition inhibiting material layer is not present. Thepatterned deposition inhibitor can be provided by uniformly coating adeposition inhibitor material and patterning using photolithography. Inpreferred embodiments, the patterned deposition inhibitor layer 810 isformed by a printing method. In other preferred embodiments, thedeposition material is polyvinyl pyrrolidone (PVP). The pattern of thefirst patterned deposition inhibitor 810 has a contact via portion 822over the pad of the TFT gate 120 and a gate via portion 824 in theregion of the gate where the channel of the TFT will be formed. Both thecontact via portion 822 and the gate via portion 824 are formed by theportions of first pattern where the deposition inhibitor is present.

Next, as shown in FIGS. 10a and 10b , a first patterned dielectric layer130 is obtained by selectively depositing an inorganic dielectricmaterial in the region 820 of the substrate where inhibitor is notpresent using an atomic layer deposition process. The first patterneddielectric layer 130 covers a portion of the gate 120 of the TFT, andmay be of any pattern that has this feature. As shown the firstpatterned dielectric layer 130 has a contact via 812 and a gate via 814.The dielectric layer can be of any composition, but is preferably ametal oxide, more preferably Al₂O₃. The atomic layer deposition processis preferably a spatial ALD process. As indicated above, when using SADthe inorganic layer has the same pattern as the inhibitor used topattern it, with the occupied regions of the pattern inverted. As such,the first patterned dielectric layer 130 has the same first pattern asthe first patterned inhibitor layer 810. In particular the patterneddielectric layer 130 is formed by growing the dielectric material in theregions 820 of the first patterned inhibitor layer 810 where theinhibitor is absent.

Following the selective deposition of the first patterned dielectriclayer 130, the first patterned deposition inhibitor layer 810 is removedas in Step 25. The result of Step 25 is shown in FIG. 11a and 11b . Inone preferred embodiment, the first patterned deposition inhibitor layer810 is removed by simultaneously treating the first depositioninhibiting material layer 810 and the first patterned dielectric layer130. The removal can be accomplished by a wet process, such as a solventrinse, or a dry process such as a plasma process or UV-ozone process.The removal preferably uses a plasma process, more probably an O₂ plasmaprocess.

Next, a second patterned deposition inhibitor layer 830 is provided onsubstrate 110 in Step 35, as shown in FIGS. 12a and 12b . Preferably,the second patterned deposition inhibitor layer 830 is provided usingthe same method as the first patterned deposition inhibitor layer 810.The second patterned deposition inhibitor layer 830 has a second patternthat is different from the first pattern of the first patterneddeposition inhibitor layer 810. The second patterned depositioninhibitor layer 830 and the first patterned deposition inhibitor layer810 have a common feature of a contact via portion 842, having a contactvia portion is required in all layers of the variable thicknessdielectric layer 150 where contact between different conductive layersis desired. As shown in FIGS. 12a and 12b , the second pattern does nothave a gate via 814, and instead a portion of the regions 840 where thedeposition inhibitor layer is not present is located over the gate 120.

Next, as shown in FIGS. 13a and 13b , a patterned second inorganic thinfilm dielectric layer 140 is formed as in Step 40 by selectivelydepositing an inorganic dielectric material in the region 840 of thesubstrate 110 where the second deposition inhibitor material is notpresent using an atomic layer deposition process. The second patterneddielectric layer 140 has the second pattern of the second depositioninhibitor layer 830. As noted above, the second pattern and the firstpatterns are different. The second patterned dielectric layer 140 canhave the same or different material composition than the first patterneddielectric layer 130. Preferably the first patterned dielectric layer130 and the second patterned dielectric layer 140 are formed from thesame material, and have the same material composition. Preferably, thesecond patterned dielectric layer 140 is an insulating metal oxide,preferably alumina. The variable thickness dielectric stack 150 as shownis made up of first patterned dielectric layer 130 and second patterneddielectric layer 140. The first patterned dielectric layer 130 andsecond patterned dielectric layer 140 can have the same layer thickness,or different thicknesses. One or both of the first patterned dielectriclayer 130 and second patterned dielectric layer 140 can be formed ofmultiple individual layers having the same pattern by performing SADpatterning steps. In alternative embodiments, additional patterneddielectric layers having patterns that are different from either thefirst or second patterns can be used in forming variable thicknessdielectric stack 150.

Following the selective deposition of the second patterned dielectriclayer 140, the second patterned deposition inhibitor layer 830 isremoved as in Step 50. The resultant structure of the inhibitor removalis shown in FIGS. 14a and 14b . The removal processes should beunderstood from the previous descriptions, and is preferably an O₂plasma process.

Next, a patterned semiconductor layer is formed in Step 765. Thepatterned semiconductor layer 170 can be formed using any method knownin the art, including depositing a uniform layer of semiconductormaterial and patterning using photolithography. The semiconductor ispreferably a metal oxide semiconductor, preferably an n-type metal oxidesemiconductor. Preferably the n-type metal oxide semiconductor is aZnO-base material, ZnO or IGZO for example. In some embodiments thepatterned semiconductor layer can be formed using a selective areadeposition process. When using a selective area deposition process, athird patterned deposition inhibitor layer 850 is provided on substrate110 and contains regions 860 where the deposition inhibitor material isnot present as shown in FIGS. 15a and 15b . Preferably, the thirdpatterned deposition inhibitor layer 850 is provided using the samemethod as the first patterned deposition inhibitor layer 810.

As shown in FIGS. 16a and 16b the patterned semiconductor layer 170, isobtained by selectively depositing an inorganic semiconductor materialin the region 860 of the third deposition inhibitor layer 850 where thedeposition inhibitor is not present using an atomic layer depositionprocess. As shown, the patterned semiconductor layer 170 has the samepattern as the third patterned deposition inhibitor layer 850. Followingthe selective deposition of the second patterned dielectric layer 140,the third patterned deposition inhibitor layer 850 is removed. Theresultant structure of the inhibitor removal is shown in FIGS. 17a and17b . The removal processes should be understood from the previousdescriptions, and is preferably an O₂ plasma process. The structureshown in FIGS. 17a and 17b is the result of process Step 765, formingthe patterned semiconductor layer 170.

In some alternative embodiments, the third deposition layer 850 is usedto form a patterned third dielectric layer, or a buffer layer, as shownin TFT 102 of FIG. 2. The patterned dielectric buffer layer 160 isformed after forming the third deposition inhibitor layer 850 and priorto selectively depositing the semiconductor layer 170 by selectivelydepositing a dielectric material using an atomic layer depositionprocess (not shown). The patterned dielectric buffer layer 160 has thesame pattern as the patterned semiconductor layer 170, as shown in FIG.2.

Next, a second patterned conductive layer is formed in Step 770 to formsource and drain electrodes 180,185 in contact with the semiconductorlayer 170. The source and drain electrodes 180,185 can be formed usingany method known in the art, including depositing a uniform layer ofconductive material and patterning using photolithography. The sourceand drain electrodes 180,185 can be formed of any conductive materialincluding metals or conducting metal oxides. In some embodiments thesource and drain electrodes 180,185 can be formed using a selective areadeposition process. When using a selective area deposition process, afourth patterned deposition inhibitor layer 870 is provided on substrate110 and contains regions 880 where the deposition inhibitor is notpresent as shown in FIGS. 18a and 18b . Preferably, the fourth patterneddeposition inhibitor layer 870 is provided using the same method as thefirst patterned deposition inhibitor layer 810.

As shown in FIGS. 19a and 19b , the source and drain electrodes 180,185are obtained by selectively depositing an inorganic conductor materialin the region 860 of the fourth deposition inhibitor layer 870 where thedeposition inhibitor is not present using an atomic layer depositionprocess. As shown, the source and drain electrodes 180,185 have the samepattern as the fourth patterned deposition inhibitor layer 870. In thisembodiment, the second patterned conductive layer includes a portionthat contacts the gate 120 through the contact via formed in themultilayer dielectric layer. Following the selective deposition of thesource and drain electrodes 180,185, the fourth patterned depositioninhibitor layer 850 is removed. The resultant structure after inhibitorremoval is shown in FIGS. 20a and 20b . The removal processes should beunderstood from the previous descriptions, and is preferably an O₂plasma process. The structure shown in FIGS. 20a and 20b is equivalentto the structure shown in FIGS. 1a and 1b , and is the result of processStep 770, forming the source and drain electrodes 180,185.

FIG. 21 illustrates an equivalent circuit for an all-enhancement-modeinverter. An all enhancement-mode inverter includes the first and secondtransistors T1 and T2. The first transistor T1 functions as a loadtransistor, and the second transistor T2 functions as a switching ordrive transistor. Both the first TFT T1 and the second TFT T2 areoperated in an enhancement mode.

As described above, an enhancement mode device is one that is normallyoff, and has a threshold voltage greater than zero. Each of the firstand second transistors T1 and T2 includes a gate, a gate dielectriclayer, a semiconductor layer, and source and drain electrodes. Thesemiconductor layer may be made of a semiconducting metal oxidematerial, and is preferably a ZnO-based semiconductor, for example, butnot limited to, ZnO or InGaZnO4. As shown in an all-enhancement-modeinverter, the source of the load transistor T1 is electrically connectedto the drain of the drive transistor T2, and the drain and gate of theload transistor T1 are electrically connected.

The flow shown in previously discussed FIG. 7 can be used as describedto build all-enhancement-mode inverters, and circuits containing them.FIGS. 22a and 22b illustrate the result of using a variable thicknessdielectric layer as formed in Step 60, to build enhancement-modeinverters where tuning the relative current-carrying ability of thedrive and load TFTs can be accomplished using the gate dielectricthickness of each TFT in addition to the W/L dimensions of theirchannels. As shown, FIG. 22b is a cross-sectional view of the plan viewof FIG. 22a taken along the line A-A′. The specific layout of theinverter is shown in FIGS. 22a and 22b is chosen for simplicity ofillustration. Many designs are possible and are within the scope of thecurrent invention as long as they meet the requirements of having avariable thickness dielectric stack 150 and are properly connected asthe equivalent circuit diagram of FIG. 21.

As shown in FIGS. 22a and 22b , an enhancement-mode inverter 700 of thepresent invention has a load transistor 205 having a bottom gatearchitecture with a first source 280, a first drain 285, a load channelregion, a load gate dielectric 255 in the load channel region having aload dielectric thickness, a first semiconductor layer 270, and a firstgate electrode 220. The drive transistor 200 of the enhancement-modeinverter 700 of the present invention has a bottom gate architecturewith a second source 180, a second drain 185, a drive channel region, adrive gate dielectric 155 in the drive channel region having a drivedielectric thickness less than the load dielectric thickness, a secondsemiconductor layer 170 and a second gate electrode 120. The drive andload transistors 200, 205 are connected as in the equivalent circuitshown in FIG. 21; the first source 280 is electrically connected to thesecond drain 185 and the first gate 220 is electrically connected to thefirst drain 285. The drive and load TFTs 200,205 have a common shareddielectric stack 152 that is a variable thickness dielectric stack 152of the present invention. As shown in FIGS. 22a and 22b , the variablethickness dielectric stack 152 includes optional dielectric buffer layer160, a first dielectric layer 130 having a gate via 814 over the gate120 of the drive TFT 200, and a second dielectric layer 140 that has auniform thickness in the channel region of both the drive TFT 200 andthe load TFT 205. As shown in FIGS. 22a and 22b , the common shareddielectric stack 152 is in contact with the second gate and has first,second, and third regions, the first region being the drive gatedielectric 155 which is in contact with the second semiconductor layer170 in the drive channel region, the second region is adjacent to thefirst region and has the drive dielectric thickness, and the thirdregion adjacent to the second region and has the load dielectricthickness. As shown in FIGS. 22a and 22b , the drive and load TFTs 200,205 can be formed simultaneously and have their individual elementsformed in common layers. As shown, the first semiconductor layer 270 andthe second semiconductor layer 170 are parts of a common semiconductorlayer. The first gate 220 and the second gate 120 are parts of a commonfirst electrically conductive layer. The first source 280, the firstdrain 285, the second source 180, and the second drain 185 are parts ofa common second electrically conductive layer. The variable thicknessdielectric layer 152 has contact vias to enable the electrical andphysical connections of the inverter 700.

The use of a variable thickness common dielectric stack shown as shownin FIGS. 22a and 22b has the advantage of providing another dimension insizing and optimizing the TFT size in optimizing the inverter andcircuit design. Unlike standard circuit designs, which are limited tousing the channel dimensions of the TFTs, the use of the variablethickness common dielectric stack allows the performance of the driveand load TFTs 200,205 to be independently tuned using the thickness ofthe drive gate dielectric 155 and the load gate dielectric 255.

Some embodiments of the present invention relate to top-gate thin filmtransistors (TFTs) which, as previously described, have a semiconductorlayer that is between the substrate and the gate layer. Unlike bottomgate TFTs, top gate TFTs are inherently protected from the environmentsince the semiconductor layer is between the substrate and the gatedielectric. When forming top gate TFTs on some substrates, includinginorganic substrates, the back-channel interface properties can causethe device to operate in the depletion mode.

FIG. 23a is a cross-sectional diagram of a prior art top gate TFT 500,taken along the line A-A′ of the plan view shown in FIG. 23c . The TFT500 shown in FIG. 13a is a top-gate structure that is representative ofany top gate TFT 500, having source and drain electrodes 580,585 incontact with a substrate 510, a semiconductor layer 570 that is over andin contact with the source/drain 580/585, a dielectric layer 530 overand in contact with semiconductor layer 570, and a gate 520 over and incontact with the dielectric layer 520, a so-called staggered structure.FIG. 23b illustrates a different embodiment of a typical top gate TFTwhere the semiconductor layer 570 is under the source and drainelectrodes. The remainder of the device is constructed as shown in FIG.23a . The substrate 510 can be any previously discussed substrate, andmay contain a plurality of predefined layers. The substrate 510 ispreferably an inorganic substrate. The source and drain haveconventionally accepted meanings, and either electrode shown may bedesignated the source (or drain) as is required by the application orcircuit. The source and drain electrodes 580,585 can be a singleconductive material, as shown in FIGS. 23a and 23b or may comprise anynumber of conductive material layers. The gap between the source anddrain electrodes 580, 585 defines the channel of TFT 500, as isconventionally understood. The inorganic semiconductor layer 570 is incontact with both the source and drain electrodes 580, 585 and thesubstrate 510, as shown in FIG. 23a . The inorganic semiconductor layer570 can be a metal oxide, for example a ZnO-based material. Thedielectric layer 530 can be a single layer of dielectric, as shown, orin embodiments of the present invention the dielectric layer 530 isformed from a variable thickness dielectric layer. The depletion-modeperformance of the top gate TFT is useful in combination with theenhancement mode bottom gate TFTs to constructenhancement-depletion-mode inverters and circuits.

In FIG. 24, an equivalent circuit for an enhancement-depletion-modeinverter is shown. The inverter includes the first and secondtransistors T1 and T2, which each include the oxide semiconductor. Thefirst transistor T1 functions as a load transistor, and the secondtransistor T2 functions as a switching or drive transistor. The firsttransistor T1 is operated in a depletion mode and has a top gatearchitecture. The second transistor T2 is operated in enhancement modeand has a bottom gate architecture.

The depletion mode operation of the first transistor T1 implies that thethreshold voltage is shifted in a negative direction with respect to thethreshold of the drive transistor T2. Each of the first and secondtransistors T1 and T2 includes a gate, a gate dielectric layer, asemiconductor layer, and source and drain electrodes. The semiconductorlayer may be made of a semiconducting metal oxide material, and ispreferably a ZnO-based semiconductor, for example, but not limited to,ZnO or InGaZnO4. As shown in an enhancement-depletion-mode inverter, thesource of the load transistor T1 is electrically connected its own gate,and to the drain of the drive transistor T2.

As shown in FIGS. 25a and 25b , an enhancement-depletion-mode inverter705 of the present invention has a load transistor 500 having a top gatearchitecture with a first source 580, a first drain 585, a load channelregion, a load gate dielectric 555 in the load channel region having aload dielectric thickness, a first semiconductor layer 570, and a firstgate electrode 520. The drive transistor 104 of theenhancement-depletion-mode inverter 705 of the present invention has abottom gate architecture with a second source 180, a second drain 185,drive channel region, a drive gate dielectric 157 in the drive channelregion having a drive dielectric thickness different than the loaddielectric thickness, a second semiconductor layer 170 and a second gateelectrode 120. The drive and load transistors 104, 500 are connected asin the equivalent circuit shown in FIG. 24; the first source 580 iselectrically connected to the second drain 185 and the first source 580is electrically connected to the first gate 520.

The drive and load TFTs 104, 500 have a common shared dielectric stack154. As shown in FIGS. 25a and 25b , the common dielectric stack 154includes a first dielectric layer 135 and a second dielectric layer 145that both have a uniform thickness in the channel regions of both thedrive TFT 104 and the load TFT 500. The drive transistor 104 has theoptional dielectric buffer layer 160, and has a thicker drive gatedielectric 157 than the load gate dielectric 555.

FIGS. 26a and 26b illustrate an enhancement-depletion-mode inverter 706of the present invention with a load transistor and a drive transistorthat share a common dielectric layer that is the variable thicknessdielectric layer 150 of the current invention. The load transistor 500has a top gate architecture with a first source 580, a first drain 585,a load channel region, a load gate dielectric 555 in the load channelregion having a load dielectric thickness, a first semiconductor layer570, and a first gate electrode 520 and a drive transistor 100 having abottom gate architecture with a second source 180, a second drain 185,drive channel region, a drive gate dielectric 156 in the drive channelregion having a drive dielectric thickness different than the loaddielectric thickness, a second semiconductor layer 170 and a second gateelectrode 120. The drive and load transistors 100, 500 are connected asin the equivalent circuit shown in FIG. 24; the first source 580 iselectrically connected to the second drain 185 and the first source 580is electrically connected to the first gate 520.

The drive and load TFTs 100, 500 have a common shared dielectric stack150. As shown in FIGS. 26a and 26b , the dielectric stack 150 is avariable thickness dielectric stack 150 of the present invention. Asshown in FIGS. 26a and 26b , the variable thickness dielectric stack 150has a first dielectric layer 130 having a gate via 814 over the gate 120of the drive TFT 100, and a second dielectric layer 140 that has auniform thickness in the channel region of both the drive TFT 100 andthe load TFT 500. As shown in FIGS. 26a and 26b , the common shareddielectric stack 150 is in contact with the second gate and has first,second, and third regions, the first region being the drive gatedielectric 156 that is in contact with the second semiconductor layer170 in the drive channel region, the second region adjacent to the firstregion and having the drive dielectric thickness, and the third regionadjacent to the second region and having load dielectric thickness.

The use of a variable thickness common dielectric stack shown in FIGS.25a and 25b and 26a and 26b has the advantage of providing anotherdimension in sizing and optimizing the TFT size in optimizing theinverter and circuit design. Unlike standard circuit designs, which arelimited to using the channel dimensions of the TFTs, the use of thevariable thickness common dielectric stack allows the performance of thedrive and load TFTs to be independently tuned using the thickness of thedrive gate dielectric and the load gate dielectric. Additionally, byusing a top gate architecture for the depletion-mode load TFT with thesame semiconductor material as the bottom gate drive TFT, as well ascommon layers allows for simple manufacturing of such circuits. As shownin FIGS. 25a and 25b and 26a and 26b , the drive and load of anenhancement-depletion-mode inverter can be formed simultaneously andhave their individual elements formed in common layers.

FIG. 27 is a Step diagram having a process flow derived from that ofFIG. 5 to form an enhancement-depletion-mode inverter with a staggeredbottom gate drive TFT and a top gate load TFT that share a commonvariable thickness dielectric layer, as illustrated in FIGS. 26a and 26b. FIG. 27 serves to further illustrate the process flow of FIG. 5, withadditional steps useful in forming any thin film transistor structure,and also to fabricate multiple architectures on a single substrate. Byappropriately choosing the pattern of each layer, top and bottom gatetransistors can easily be built using the same process. FIG. 27 includesall of the steps of FIG. 5, namely Step 1 providing a substrate, Step 10forming a first patterned conductive layer, Step 60, forming a variablethickness dielectric stack, as defined by Steps 15 through 50, and Step70 forming a second patterned conductive layer which should beunderstood from the previous descriptions. FIG. 27 includes theadditional steps of forming a first patterned semiconductor layer inStep 750, and forming a second patterned semiconductor layer in step760.

When using the process flow shown in FIG. 27 to fabricateenhancement-depletion mode inverter illustrated in FIG. 26, the firstpatterned conductive layer formed in Step 10 forms the gate 120 of thedrive TFT, and the source and drain electrode 580, 585 of the load TFT.The first patterned semiconductor layer 570 can be formed either beforethe first conductive layer (as shown in FIG. 27), or alternatively afterforming the first conductive layer. The first patterned semiconductorlayer 570 is the semiconductor layer for the load TFT, and is in contactwith the source and drain electrodes 580, 585 formed in Step 10.

The variable thickness dielectric stack formed in Step 60 is common toboth the drive TFT and the load TFT. The variable thickness dielectricstack formed such that the drive gate dielectric 156 of the drive TFT isthinner than the load gate dielectric 555 thickness. After forming thevariable thickness dielectric stack 150, a second semiconductor layer170 is formed in Step 765. The second semiconductor layer 170 is thesemiconductor layer 170 for the drive TFT 100, and is in contact withthe source and drain electrodes 180,185 that are formed in Step 70. Step70 forms the second conductive layer which contains the source and drainelectrodes 180,185 of drive TFT 100 and the gate 520 of the load TFT500. The second conductive layer can be formed either before or afterthe second semiconductor layer 170, depending on the desiredarchitecture of the drive TFT.

Each transistor formed using the process of FIG. 27 has a channel regionand a region where the source and the drain electrode overlap at least aportion of the gate to form an overlap region, this overlap region beingadjacent to the channel region. The overlap region of the drive TFT hasat least two portions: a first portion of the overlap region thatincludes both the first and the second dielectric layers, and a secondportion of the overlap region which includes only one of the first orsecond dielectric layers. The second portion of the overlap region ofthe drive TFT is adjacent to the channel region of the drive TFT, andthe second portion of the overlap region is between the channel regionof the drive TFT and the first portion of the overlap region of thedrive TFT. The channel region and the second portion of the overlapregion have the same thickness. The first portion of the overlap regionand includes both the first and the second dielectric layers, and isfurther from the channel to prevent against the source or drainelectrode shorting to the gate. In contrast, the overlap region of theload TFT only as a single region that includes both the first and seconddielectric layers. The thickness of the dielectric layer having thechannel via can be used to independently tune the gate dielectricthickness of the drive and load TFTs.

EXAMPLES General Conditions for the Preparation of Layers UsingAtmospheric Pressure ALD

The preparation of a thin film coating of the material layers on glasssubstrates as used in the examples is described below. The ALD coatingdevice used to prepare these layers, namely aluminum oxide, ZnO:N, andAl-doped ZnO (AZO), has been described in detail in US PatentApplication Publication No. US 2009/0130858, the disclosure of which isherein incorporated by reference in its entirety. The coating device hasan output face (facing up) that contains spatially separated elongatedgas channels and operates on a gas bearing principle. The coating devicecan be understood with respect to delivery head 900 shown in FIG. 28.Each gas channel is composed of an output slot 95, 93, 92 which suppliesgas to the output face 905, and adjacent exhaust slots 91 which removegas from the output face 905. The order of the gas channels isP-O-P-M-P-O-P-M-P-O-P where P represents a purge channel, O represents achannel containing an oxygen based precursor, and M represents a channelcontaining a metal based precursor. As a substrate moves relative to thecoating head it sees the above sequence of gases which results in ALDdeposition.

A 2.5 by 2.5 inch square (62.5 mm square) glass substrate attached to aheated backer is positioned above the output face of the coating deviceand is maintained in close proximity to the output face by anequilibrium between the pull of gravity, the flow of the gases suppliedto the output face, and a slight amount of vacuum produced at theexhaust slot. For all of the examples, the exhaust slot pressure wasapproximately 40 inches of water below atmospheric pressure. The purgegas P is composed of pure nitrogen. The oxygen reactive precursor O is amixture of nitrogen, water vapor, and optionally ammonia vapor. Themetal reactive precursor M is one or a mixture of active metal alkylsvapor in nitrogen.

The metal alkyl precursors used in these examples were dimethylaluminumisopropoxide (DMAI) and diethyl zinc (DEZ). The flow rate of the activemetal alkyl vapor was controlled by bubbling nitrogen through the pureliquid precursor contained in an airtight bubbler by means of individualmass flow control meters. This saturated stream of metal alkyl was mixedwith a dilution flow before being supplied to the coating device. Theflow of water vapor was controlled by adjusting the bubbling rate ofnitrogen passed through pure water in a bubbler. This saturated streamof water vapor was mixed with a dilution flow before being supplied tothe coating device. The flow of ammonia vapor was controlled by passingpure ammonia vapor from a compressed fluid tank through a mass flowcontroller and mixing with the water vapor stream. All bubblers wereheld at room temperature. The temperature of the coating was establishedby controlling heating of both the coating device and the backer to adesired temperature. Experimentally, the flow rates of the individualgasses were adjusted to the settings shown in Table 1 for each of thematerial layers coated in the examples contained herein. The flows shownare the total flows supplied to the coating device, and thus arepartitioned equally among the individual gas channels.

The coating process was then initiated by oscillating the substrateacross the coating head for the number of cycles necessary to obtain auniform deposited film of the desired thickness for the given example.The coating head as described above contains two full ALD cycles (twooxygen and two metal exposures per single direction pass over the head),therefore a round trip oscillation represents 4 ALD cycles. All sampleswere coated at a substrate temperature of 200° C.

TABLE 1 N₂ dilution N₂ with dilution N₂ Metal with Inert Residence DMAIDEZ NH3 Water Alkyl water Purge Time Layer flow (seem) flow (seem) flow(seem) flow (seem) (seem) (seem) (seem) (ms) Al₂O₃ 65 0 0 65 1500 22503000 50 ZnO:N 0 60 4 45 1500 2250 3000 50 AZO 10 30 0 22.5 1500 22503000 50

Bottom Gate TFT Experiments to Probe the Variable Thickness DielectricLayer

In order to probe the impact of the variable thickness dielectric layeron transistor performance and stability, staggered (i.e. “top contact”)bottom-gate TFTs were built using the spatial atomic layer depositiontool described above. The Example TFTs were fabricated as describedbelow, and the relative differences between the samples are called outin the following descriptions for clarity. Inventive TFTs werefabricated using the process flow described in FIG. 7. Electricaltesting of the transistors was accomplished by using a probe station tocontact the gate and source/drain electrodes. The transistors were sweptin the saturation regime, with the drain being held constant at 14 V(Vd=14 V), and the gate voltage was swept over the range indicated.

Comparative Example C1 Bottom Gate TFT with Standard 500 Å Dielectric

Comparative Example C1 is a bottom-gate device having a constantthickness dielectric stack. Comparative Example C1 was fabricated usingthe combination of spatial ALD and selective area deposition (SAD). Thegate layer was 1000 Å of AZO, the gate dielectric was 500 Å of Al₂O₃,200 Å of ZnO:N was used for the semiconductor layer and 1000 Å AZO wasused for the source and drain electrodes. The printed inhibitor used topattern each of the device layers was a 2 wt % polyvinyl pyrrolidone(PVP) k30 in diacetone alcohol solution which was printed using aDimatix 2500 printer. The printing was done using the 10 pL cartridgewhich resulted in a spot size on the substrate of approximately 90microns under typical operating conditions. The patterns were printed ata pixel-to-pixel distance of 70 microns (363 dpi). The pattern of PVPused to pattern the AZO layer in the source and drain pattern was lefton the device during testing, such that the back channel of thesemiconductor layer was covered by a thin layer of PVP k30. Thetransistor had as designed dimensions of W=6 pixel/L=1 pixel. Theperformance data for Comparative Example C1 can be found in the Id-Vgcurve shown in FIG. 29.

Inventive Example I1 Bottom Gate TFT with an Extra 250 Å Outside of theChannel Region

Inventive Example I1 is a bottom-gate transistor with the variablethickness dielectric stack of the present invention. Inventive ExampleI1 was fabricated as Comparative Example C1 with the followingexceptions. After removing the PVP used to pattern the gate, a 250 Åextra dielectric layer having a gate via was formed using selective areadeposition. The transistors were completed and characterized as incomparative example C1. The performance data for Inventive Example I1can be found in the Id-Vg curve shown in FIG. 29.

Inventive Example I1 Bottom Gate TFT with an Extra 500 Å Outside of theChannel Region

Inventive Example I1 is a bottom-gate transistor with the variablethickness dielectric stack of the present invention. Inventive ExampleI1 was fabricated as Comparative Example C1 with the followingexceptions. After removing the PVP used to pattern the gate, a 500 Åextra dielectric layer having a gate via was formed using selective areadeposition. The transistors were completed and characterized as incomparative example C1. The performance data for Inventive Example I2can be found in the Id-Vg curve shown in FIG. 29.

As can be seen in FIG. 29, the inventive examples having the extradielectric outside of the channel region perform similarly to thecomparative example without the extra dielectric layer, having the samegate dielectric thickness within the channel region.

All-Enhancement-Mode Inverters

In order to further probe the usefulness of the variable thicknessdielectric stack, all enhancement-mode inverters were fabricated in fivestage ring oscillator circuits. The Example circuits were fabricated asdescribed below, and the relative differences between the samples arecalled out in the following descriptions for clarity. Inventive invertercircuits were fabricated using the process flow described in FIG. 7.

Comparative Example C2 Inverter with Fixed Thickness Dielectric Stack

Comparative example C2 was prepared generally as described above forcomparative example C1 and each of the transistors of C2 used the sameprocess as comparative example C1. However, comparative example C2 usedpatterns to form all-enhancement-mode inverters arranged into afive-stage ring oscillator. Each transistor in each of the inverters hadthe same dielectric thickness, 500 Å, as in comparative example C1. Thedrive transistors for each inverter had as designed W=6 pixel/L=1 pixel,and the load transistors for each inverter had an as designed W=3pixel/L=3 pixel. The sample was analyzed by using a probe station tocontact the AZO Vdd, ground, and output nodes of the ring oscillator,and the oscillator was characterized as a function of Vdd. Thefrequency, time per stage, and fraction of the peak-to-peak swing forVdd=16 V can be found in Table 2.

Comparative Example C3 Inverter with Fixed Thickness Dielectric Stack

Comparative example C3, was prepared as Comparative example C2 with thefollowing exception. The drive transistors for each inverter had asdesigned W=12 pixel/L=1 pixel, and the load transistors for eachinverter had an as designed W=4 pixel/L=3 pixel. The circuit wascharacterized as in Comparative example C2, and the frequency, time perstage, and fraction of the peak-to-peak swing for Vdd=16 V can be foundin Table 2.

Inventive Example I3 Inverter with Variable Thickness Dielectric Stack

Inventive example I3, was prepared as comparative example C1 with thefollowing exception. After removing the PVP used to pattern the gate, a250 Å extra dielectric layer having a gate via was formed usingselective area deposition (as in Inventive Example I1). The pattern ischosen so that the inverters have the variable dielectric stackillustrated in FIGS. 22a and 22b . This results in inverters ofInventive Example I3 having drive TFTs with a total gate dielectricthickness of 500 Å, and the load TFT having a total gate dielectricthickness of 750 Å. The drive transistors for each inverter had asdesigned W=6 pixel/L=1 pixel, and the load transistors for each inverterhad an as designed W=3 pixel/L=3 pixel. The circuit was characterized asin Comparative example C2, and the frequency, time per stage, andfraction of the peak-to-peak swing for Vdd=16 V can be found in Table 2.

Inventive Example I4 Inverter with Variable Thickness Dielectric Stack

Inventive example I4, was prepared as comparative example I3. The drivetransistors for each inverter had as designed W=6 pixel/L=1 pixel, andthe load transistors for each inverter had an as designed W=3 pixel/L=1pixel. The circuit was characterized as in Comparative example C2, andthe frequency, time per stage, and fraction of the peak-to-peak swingfor Vdd=16 V can be found in Table 2.

Inventive Example I5 Inverter with Variable Thickness Dielectric Stack

Inventive example I5, was prepared as comparative example I3. The drivetransistors for each inverter had as designed W=12 pixel/L=1 pixel, andthe load transistors for each inverter had an as designed W=4 pixel/L=1pixel. The circuit was characterized as in Comparative example C2, andthe frequency, time per stage, and fraction of the peak-to-peak swingfor Vdd=16 V can be found in Table 2.

TABLE 2 Drive Load Drive Load TFT TFT time/ TFT TFT DielectricDielectric stage Freq Sample W/L W/L (Å) (Å) (ms) (Hz) fraction C2 6/13/3 500 500 111.4 1795 0.328 C3 12/1  4/3 500 500 129.4 1546 0.395 I36/1 3/3 500 750 152.0 1315 0.41 I4 6/1 3/1 500 750 93.8 2132 0.33 I512/1  4/1 500 750 123.6 1618 0.41

As can be seen in Table 2, inverters and circuits containing thevariable thickness dielectric stack of the present invention can befabricated. The variable dielectric stack provides the ability toindependently select the gate dielectric thickness of the drive and loadTFTs in an inverter. Comparing C2 and I3, for inverters with the driveand load TFT with the same channel dimensions, an increase in thedielectric thickness of the load TFT results in a slower inverter andring oscillator due to the reduction in the saturation current of theload TFT. As shown in I4, this lower current can be traded off with thelength of the load TFT to form a faster circuit with a smallerfootprint. The comparison between C3 and I5 also illustrates the tradeoff of the load transistor gate dielectric thickness with for a smallerfootprint. Using inverters with the variable thickness dielectric stack,circuits layouts can be optimized simultaneously for footprint andperformance.

Enhancement-Depletion-Mode Inverters

In order to further probe the usefulness of theenhancement-depletion-mode inverters of the present invention,enhancement-depletion-mode inverters were fabricated in five stage ringoscillator circuits. The Example circuits were fabricated as describedbelow, and the relative differences between the samples are called outin the following descriptions for clarity. In order to further probe theusefulness of the variable thickness dielectric stack, InventiveEnhancement-depletion-mode inverters were fabricated using the processflow described in FIG. 27.

Inventive Example I6 Enhancement-Depletion-Mode Inverter with CommonDielectric Stack

Inventive Example I6 was fabricated using the combination of spatial ALDand selective area deposition (SAD) on a glass substrate. A 200 Å ZnO:Nsemiconductor layer for the top gate load TFT was formed first, so thatthe back channel was in contact with the glass. Next, the firstconductive layer containing the gate of the bottom gate drive TFT andthe source and drain for the load TFT was formed using 1000 Å AZO. A 400Å common dielectric layer was formed from two separately patterned 200 Ådielectric layers of Al₂O₃ to make the gate dielectric for the load TFTand a portion of the gate dielectric for the drive TFT. Next, a bufferlayer and the semiconductor layer of the drive TFT were formed from asingle inhibitor pattern with 100 Å of Al₂O₃ and 200 Å of ZnO:N. Thesecond conductive layer containing the source and drain of the bottomgate drive TFT and the gate for the load TFT was formed using 1000 ÅAZO. The printed inhibitor used to pattern each of the device layers wasa 2 wt % polyvinyl pyrrolidone (PVP) k30 in diacetone alcohol solutionwhich was printed using a Dimatix 2500 inkjet printer. All device layerswere deposited using SALD and the conditions listed in Table 1. Thepattern is chosen so that the inverters have the common dielectric stackillustrated in FIGS. 25a and 25b . This results in inverters ofInventive Example I3 having drive TFTs with a total gate dielectricthickness of 500 Å, and the load TFT have a total gate dielectricthickness of 400 Å. The drive transistors for each inverter had asdesigned W=12 pixel/L=1 pixel, and the load transistors for eachinverter had an as designed W=1 pixel/L=3 pixel. The ring oscillatoralso had a buffer inverter with a drive transistor as designed W=12pixel/L=1 pixel, and a load transistor as designed W=4 pixel/L=3 pixel.The circuit was characterized as in Comparative example C2, and thefrequency, time per stage, and fraction of the peak-to-peak swing forVdd=11.5 V can be found in Table 3.

Inventive Example I7 Enhancement-Depletion-Mode Inverter with CommonDielectric Stack

Inventive example I7, was prepared as comparative example I6 with thefollowing exception. After removing the PVP used to pattern the firstconductive layer, a 500 Å extra dielectric layer having gate via for thedrive TFT was formed using selective area deposition. The commondielectric layer was 300 Å formed from two separately patterned 150 Ådielectric layers of Al₂O₃, and the buffer layer for the drive TFT was200 Å of Al₂O₃. The pattern is chosen so that the inverters have thevariable dielectric stack illustrated in FIGS. 26a and 26b , with theaddition of a dielectric buffer layer 160 with the same pattern assemiconductor layer 170. This results in inverters of Inventive ExampleI7 having drive TFTs with a total gate dielectric thickness of 500 Å,and the load TFT having a total gate dielectric thickness of 800 Å. Thecircuit design is the same as in Inventive Example I6. The circuit wascharacterized as in Comparative example C2, and the frequency, time perstage, and fraction of the peak-to-peak swing for Vdd=11.5 V and 7.5 Vcan be found in Table 3.

Inventive Example I8 Enhancement-Depletion-Mode Inverter with CommonDielectric Stack

Inventive example I8, was prepared as comparative example I7 with thefollowing exception. The circuit design had a different design; thedrive transistors for each inverter had as designed W=12 pixel/L=1pixel, and the load transistors for each inverter had an as designed W=3pixel/L=3 pixel. The circuit was characterized as in Comparative exampleC2, and the frequency, time per stage, and fraction of the peak-to-peakswing for Vdd=11.5 V and 7.5 V can be found in Table 3.

Inventive Example I9 Enhancement-Depletion-Mode Inverter with CommonDielectric Stack

Inventive example I9, was prepared as comparative example I8 with thefollowing exception. The common dielectric layer was 150 Å formed fromtwo separately patterned 75 Å dielectric layers of Al₂O₃, and the bufferlayer for the drive TFT was 100 Å of Al₂O₃. This results in inverters ofInventive Example I8 having drive TFTs with a total gate dielectricthickness of 250 Å, and the load TFT have a total gate dielectricthickness of 650 Å. The circuit design is the same as in InventiveExample I6. The circuit was characterized as in Comparative example C2,and the frequency, time per stage, and fraction of the peak-to-peakswing for Vdd=7.5 V and 11.5 V can be found in Table 3.

TABLE 3 Drive Load Load TFT TFT time/ Vdd TFT Dielectric Dielectricstage Freq Sample (V) W/L (Å) (Å) (ms) (Hz) fraction I6 11.5 1/3 500 40079 2532 0.243 I7 7.5 1/3 500 800 492 407 0.853 I7 11.5 1/3 500 800 411486 0.983 I8 7.5 3/3 500 800 308 649 0.843 I8 11.5 3/3 500 800 264 7570.983 I9 7.5 3/3 250 650 140 1426 0.917 I9 11.5 3/3 250 650 119 16800.957

As can be seen in Table 3, enhancement-depletion-mode inverters andcircuits can be formed with a common dielectric stack, where thethickness of the drive gate dielectric for the bottom gate drive TFT canbe controlled independently from the load gate dielectric of the topgate load TFT. Inventive example I6 illustrates the ability to form veryfast circuits by using a load TFT with a thinner dielectric than that ofthe drive TFT. Inventive examples I7, I8 and I9 illustrate theperformance of enhancement-depletion-mode inverters using the variablethickness dielectric stack of the present invention. The dielectricthickness differences can be adjusted modifying either the thickness ofthe common portion of the dielectric stack as shown in I8 and I9, or bychanging the thickness of the extra dielectric layer in the variablethickness dielectric stack. Using inverters with the variable thicknessdielectric stack, circuit layouts can be optimized simultaneously forfootprint and performance.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   1 providing a substrate-   10 forming a first patterned conductive layer-   15 forming a first patterned layer of deposition inhibitor-   20 depositing a first dielectric layer using ALD-   25 removing the deposition inhibitor-   35 forming a second patterned layer of deposition inhibitor-   40 depositing a first dielectric layer using ALD-   50 removing the deposition inhibitor-   60 forming a variable thickness dielectric stack-   70 forming a first patterned conductive layer-   91 vacuum-   92 metal precursor flow-   93 oxidizer-containing flow-   95 nitrogen purge flow-   97 example substrate-   98 arrow-   99 gap-   100 bottom gate TFT-   102 bottom gate TFT-   103 bottom gate TFT-   104 bottom gate TFT-   110 substrate-   120 gate-   130 first dielectric layer-   140 thin film dielectric material layer-   143 dielectric buffer layer-   145 second dielectric layer-   150 variable thickness dielectric stack-   152 variable thickness dielectric stack-   153 variable thickness dielectric stack-   155 drive gate dielectric-   156 drive gate dielectric-   157 drive gate dielectric-   160 buffer layer-   170 semiconductor layer-   180 sourced-   185 drain-   200 drive transistor-   205 load transistor-   220 first gate electrode-   255 load gate dielectric-   270 first semiconductor layer-   280 sourced-   285 drain-   425 gate length-   455 channel length-   450 channel region-   460 first region-   470 second region-   475 length-   500 top gate transistor-   505 top gate transistor-   510 substrate-   520 gate-   530 dielectric layer-   555 load gate dielectric-   570 patterned semiconductor layer-   580 source-   585 drain-   605 first pattern deposition inhibitor layer-   615 second patterned deposition inhibitor layer-   620 first patterned conductive layer-   630 patterned first dielectric layer-   640 patterned second dielectric layer-   650 variable thickness dielectric stack-   675 overlap region-   680 second patterned conductive layer-   690 first portion of the overlap region-   695 second portion of the overlap region-   700 inverter-   705 inverter-   706 inverter-   710 forming a patterned gate-   750 forming a first patterned semiconductor layer-   760 forming a second patterned semiconductor layer-   765 forming a patterned semiconductor layer-   770 forming source and drain electrodes-   810 first patterned deposition inhibiting material layer-   812 contact via-   814 gate via-   820 region where the deposition inhibiting material layer is not    present-   822 contact via portion of the deposition inhibitor layer-   824 gate via portion of the deposition inhibitor layer-   830 second patterned deposition inhibiting material layer-   840 region where the deposition inhibiting material layer is not    present-   850 third patterned deposition inhibiting material layer-   860 region where the deposition inhibiting material layer is not    present-   870 fourth patterned deposition inhibiting material layer-   880 region where the deposition inhibiting material layer is not    present-   900 delivery head-   905 output face

1. A transistor comprising: a substrate; an electrically conductive gateover the substrate, the gate having a gate length; a source electrodeand a drain electrode over the substrate, the source and drainelectrodes separated by a gap defining a channel region, the channelregion having a channel length that is less than the gate length; asemiconductor layer in contact with the source and drain electrodes, thesemiconductor layer having a semiconductor pattern; a dielectric stackhaving a portion in contact with the gate having first, second, andthird regions, the first region in contact with the semiconductor layerin the channel region and having a first thickness, the second regionadjacent to the first region and having the first thickness, and thethird region adjacent to the second region and having a thickness thatis greater than the first thickness, wherein the dielectric stackincludes a plurality of layers with one of the plurality of layers thatis in contact with the semiconductor layer having a different patternthan another of the plurality of layers, the layer of the dielectricstack that is in contact with the semiconductor layer having the samesemiconductor pattern, and the layer of the dielectric stack that is incontact with the semiconductor layer having terminating edges that arealigned to be coincident with corresponding terminating edges of thesemiconductor layer.
 2. The transistor of claim 1, wherein thesemiconductor layer is under the source electrode and under the drainelectrode.
 3. A transistor comprising: a substrate; an electricallyconductive gate over the substrate, the fate having a gate length; asource electrode and a drain electrode over the substrate, the sourceand drain electrodes separated by a gap defining a channel region, thechannel region having a channel length that is less than the gatelength; a semiconductor layer in contact with the source and drainelectrodes; a dielectric stack in contact with the gate having first,second, and third regions, the first region in contact with thesemiconductor layer in the channel region and having a first thickness,the second region adjacent to the first region and having the firstthickness, and the third region adjacent to the second region and havinga thickness that is greater than the first thickness, wherein thedielectric stack includes a plurality of layers with one of theplurality of layers that is in contact with the semiconductor layerhaving a different pattern than another of the plurality of layers, thelayer of the dielectric stack that is in contact with the semiconductorlayer having the same pattern over the substrate as the semiconductorlayer, and the layer of the dielectric stack that is in contact with thesemiconductor layer having edges that are aligned to be coincident withcorresponding edges of the semiconductor layer, wherein thesemiconductor layer is over the source electrode and over the drainelectrode.
 4. The transistor of claim 1, wherein the gate is in contactwith the substrate.
 5. The transistor of claim 1, wherein the sourceelectrode and the drain electrode are in contact with the substrate. 6.(canceled)
 7. The transistor of claim 1, wherein each of the pluralityof layers of the dielectric stack has the same material composition. 8.The transistor of claim 7, wherein there is an interface region betweeneach of the layers of the plurality of layers.
 9. The transistor ofclaim 1, wherein one of the plurality of layers includes a differentmaterial than another of the plurality of layers. 10.-11. (canceled) 12.The transistor of claim 1, wherein the different pattern includes a viain the first region and the second region.
 13. The transistor of claim1, wherein semiconductor layer has a length greater than the gatelength.